The Centre Block (French: Édifice du Centre) is the main building of the Canadian parliamentary complex on Parliament Hill, in Ottawa, Ontario, containing the House of Commons and Senate chambers, as well as the offices of a number of members of parliament, senators, and senior administration for both legislative houses. It is also the location of several ceremonial spaces, such as the Hall of Honour, the Memorial Chamber, and Confederation Hall.

Built in the Gothic Revival style, the present Centre Block is the building's second iteration. The first was destroyed by fire in 1916; all that remains of the original building is the Library of Parliament, at the rear of the Centre Block. Though construction began immediately after the blaze, sculpting work on the interior continued through the 1970s. Then, from 2018, MPs were moved elsewhere for renovations lasting until 2031.

One of the most recognizable buildings in Canada, the Centre Block is depicted on the Canadian $10 bill (the Library of Parliament), $20 bill (the Peace Tower), and the $50 bill.

Designed by Jean-Omer Marchand [fr] and John A. Pearson, the Centre Block is a symmetrical structure, 144 m (472 ft) long by 75 m (246 ft) deep, and six storeys high, built in the modern Gothic Revival style. As such, it displays a multitude of stone carvings, including gargoyles, grotesques, and friezes, keeping with the Victorian High Gothic style of the rest of the parliamentary complex. The walls are faced with more than 50,000 blocks of over 24 different types of stone, though a rustic finished Nepean sandstone is the predominant kind of masonry, with dressed stone trim around the 550 windows and other edges. The roof is of reinforced concrete covered with copper, and dotted with dormer windows. The interior walls are sheeted with Tyndall stone, a dolomitic limestone quarried in southeastern Manitoba and chosen by the architect for the richness of its vibrant colour and rich pattern formed by darker brown spots which are fossilized shallow marine mud burrows. These surfaces are augmented by sculptural decoration done in Indiana limestone.

The Centre Block houses offices and facilities, including the prime minister's office, that of the leader of His Majesty's Loyal Opposition, and the offices of other party leaders, as well as senators, ministers, and Commons staff. Further, there are numerous parliamentary committee rooms and the Parliamentary Press Gallery.

The Centre Block is arranged symmetrically around Confederation Hall, located immediately inside the main entrance. It is an octagonal chamber, the perimeter of which is divided by limestone clustered columns into eight bays of two different sizes, themselves subdivided by dark green syenite pillars. Behind these runs a vaulted ambulatory that supports the upper gallery. The arcaded arches are topped by gables sculpted to commemorate the confederated nature of Canada and they support one side of the hall's fan vaulted ceiling with carved bosses, while the other side rests on a single column in the centre of the room. This column is borne on a stone carved with an image of Neptune amongst sea lions and fish in a mythical sea. It was placed at noon on 2 July 1917, to mark the 50th anniversary of Confederation, and above it was carved the words:

1867 July 1917 On the fiftieth anniversary of the Confederation of British Colonies in North America as the Dominion of Canada the Parliament and People dedicate this Building in process of reconstruction after damage by fire as a memorial of the deeds of their Forefathers and of the valour of those Canadians who in the Great War fought for the liberties of Canada, of the Empire and of Humanity.

Around the central column is an inlaid marble floor with a 16-point windrose of Verde Antique serpentine from Roxbury, Vermont, and a swirl pattern of green serpentine from the Greek island of Tinos, embedded in Missisquoi Boulder Grey marble, from Philipsburg, Quebec. The overall pattern represents the essential element of water, alluding to Canada's motto: a mari usque ad mare (from sea to sea). The inner and outer circles of the floor are made of a Missisquoi Black marble from Philipsburg, Quebec, and white travertine from Italy, as well as Verde Antique serpentine separated by a band of Missisquoi Boulder Grey marble.

Though a design model of the room was presented as early as January 1918, Confederation Hall was the last part of the Centre Block's interior to be completed; the Missisquoi black marble base was laid on 11 August 1921 and the Tyndall limestone vault—built from a full scale wood and plaster model—was completed in December of the following year. Still, the detailed carving, the designs for which had been finalised by Cléophas Soucy in 1941, remained incomplete until 1953, due to occurrence of the Second World War. Upon completion of this work, the tympanums were adorned with the coats of arms of Canada and the provinces, each surrounded by relevant floral symbols: on the east wall thistles for Nova Scotia, Tudor roses for Prince Edward Island, and grapes and apples for New Brunswick; on the south wall lilies, maple leaves, and Tudor roses for Newfoundland and Labrador, pine cones, oak leaves, and acorns for Nunavut, and maple leaves for British Columbia; on the west wall grapevines and apples for Saskatchewan, sunflowers, corn, and wheat for Alberta, and wheat and pine cones for Manitoba; and on the west wall sunflowers, wheat, and corn for Ontario, pine cones, oak leaves, and acorns for Canada, and Tudor roses for Quebec. The remaining territorial coats of arms are located in the southeast (Northwest Territories) and southwest (Yukon) corners. The gable springers all display the coats of arms of the provincial and territorial capitals, while the gable ramparts bear symbols of Canada's fauna.

The apex stones atop each central arch are carved into figures from Canadian life, two Inuit with huskies being found on the east wall, two heads each representing the merchant marine and agriculture on the south wall, two of Canada's First Nations people on the west wall, and a lumberjack and miner with a wheel of industry on the north wall. There were originally two renditions of the sovereign's Canadian arms, one each on the north and south walls of Confederation Hall; however, the latter was reworked in 2000 by Maurice Joanisse into the above-mentioned coat of arms of the newly created territory of Nunavut.

The building's western wing contains the House of Commons chamber, along with its antechamber and lobbies for the government and opposition, on the east and west sides of the main commons space. The doors to all are of white oak trimmed with hand-wrought iron.

The chamber is 21 metres long, 16 metres wide, and has seats for 320 members of parliament and 580 persons in the upper gallery that runs around the room's second level. The overall colour scheme is in green—visible in the carpeting, bench upholstery, draperies, paint within the gilded honeycomb cork plaster work of the cove, and the stretched linen canvas over the ceiling—and is reflective of the colour used in the House of Commons of the United Kingdom since at least 1663. That canvas, sitting 14.7 m above the commons floor and designed in 1920 by the New York decorating firm Mack, Jenney and Tyler, is painted with the heraldic symbols of the Canadian, provincial, and territorial coats of arms, with medallions at the intersections of diagonal stencilled bands in an argyle pattern. Running below this, and above the cove, is a continuous gold leaf cornice created in 1919 by Ferdinand Anthony Leonard Cerracchio (1888–1964), which displays a row of gilt figures, broken at the peak of each pointed arch by cherubs holding a cartouche, and behind all of which runs a painted grapevine with Tudor roses.

On the floor, the opposing members' benches are spaced 3.96 m apart on either side of the room, a measurement said to be equivalent to two swords' length, harkening back to when English members of parliament carried swords into the chamber. Directly between, directly opposite the main door, on the chamber's axis, is the speaker's chair, made in 1921 by the English firm of Harry Hems as an exact replica of that in the British House of Commons. It is topped by a carved wood canopy bearing a rendition of the royal coat of arms of Canada sculpted in wood from the roof of the Westminster Hall, which was built in 1397; the whole was a gift from the British branch of what is today the Commonwealth Parliamentary Association. The chair has since been augmented with a hydraulic lift, lighting, writing surfaces, and, at the foot of the chair, a television screen and computer screen to aid the speaker in monitoring the process of the house. Behind the chair is a door that gives the speaker access to the speaker's corridor, which links the commons chamber to the speaker's chambers, and which is lined with portraits of past speakers of the House of Commons.

In the commons chamber's east and west walls are 12 windows topped by pointed arches with hood moulds terminated by pendant drops. The glazing within is stained glass, commissioned as a Centennial Project in 1967 by then-speaker Lucien Lamoureux. Each window contains approximately 2,000 pieces of hand-blown glass—created in Ottawa by Russell C. Goodman using medieval techniques—arranged in a Decorated Gothic style pattern designed by R. Eleanor Milne. Divided into four sections by stone mullions, the upper parts contain geometrical tracery and provincial and territorial floral emblems amongst ferns; in the tracery at the head of the windows are symbols extracted from the coats of arms of the provinces and territories.

As with other areas of the Centre Block, the commons walls are enriched with shafts, blind tracery, friezes, and a sculpture programme. The room was the last space in the building to be carved, with sculptural work only beginning in the late 1950s and continuing intermittently for the following two decades; approximately 225 blocks of varying sizes still remain uncarved. Amongst the work done are three series of stone works: The British North America Act, a set of 12 high reliefs on the east and west walls of the chamber, carved between 1978 and 1985, and illustrating through symbols and narrative themes associated with the federal and provincial responsibilities laid out in the British North America Act; Evolution of Life, a series of 14 sculptures within the spandrels of the pier-arches at the north and south ends of the House of Commons, depicting Canada's palaeontological past and the evolution of humanity through philosophy, science, and the imagination; and Speakers and Clerks, comprising four heads carved on the jambs of the two doors on either side of the Speaker's chair, depicting the speakers and clerks of the House of Commons at the time of the opening of both parliament buildings in 1867 and 1920, respectively.

With the closure of Centre Block for renovations in December 2018, the Commons chamber was relocated to the nearby West Block.

Directly south of the House of Commons is that room's foyer, a rectangular, two storey arcaded hall surrounded by clustered limestone piers and moulded arches that support an upper cloister lined with black marble posts. The foyer is approached from Confederation Hall by the South Corridor, which is lined with portraits of former prime ministers, as well as via an entrance hall that opens to the front of the Centre Block and which is separated from the foyer by a Rose Tavernelle marble balustrade. The floor of the commons foyer is of Missisquoi Boulder Grey marble with borders of Verde Antique serpentine.

The walls bear a series of 10 bas-relief panels showing 25,000 years of Canada's history and, directly above, is a stone tracery ceiling with soft green coloured glass infill manufactured by the N.T. Lyon Glass Company of Toronto. The ceiling's appearance is similar to that found in the Senate foyer, except here the borders are of oak leaves, pine cones, and acorns; the octagonal panels are filled with Tudor roses and fleur-de-lis; the monogram HC (for House of Commons) is placed in all the small oblong panels; and the oval panels bear symbols of the various government departments amid sprigs of maple leaves: a steam crane represents Public Works; a beaver and bees represent trade and industry; a lighthouse and ship represent the naval services and a military ship at sea represents the overseas military; letters, stamps, and caduceus represent the Postmaster General; fish and an anchor represent the fisheries; wheat and a sickle represent agriculture; pounds and dollars represent finance; a crowned globe represents the Geological Survey; a moose and crown represents the Ministry of the Interior; a helmet and weapons represent militia and defence; crates and 33⅓% represent customs; a steam locomotive represents the railway; a scale and sword represents justice; picks, shovels, and a saw represent mining and forestry; and a crown, mace, and the granting of Royal Assent represent parliament itself.

The Railway Committee Room is situated to the east of and accessed from the Hall of Honour. It is a double height space done in a Beaux-Arts style, with a heavily coffered ceiling and, above a one story high, ashlar stone base, pilasters on the walls—all the aforementioned painted in a cream colour—between which are panels of moss green fabric, except where there are windows on the east wall. In it hangs a reproduction of Robert Harris' painting The Fathers of Confederation, as well as a rendition of the Canadian Royal Arms and the Will Longstaff painting The Ghosts of Vimy Ridge which commemorates the Battle of Vimy Ridge.

It is used by the official Opposition party for meetings, as well as by various committees. The current room dates from 1916 and replaced the same that existed in the 1866 Centre Block and was named for the 166 member Railway Committee that once met in it. That space also served as home to the Supreme Court of Canada from 1876 to 1889.

Directly across the Hall of Honour is the Reading Room, where the governing party's caucus meets. It is in a similar architectural style to the Railway Room, except between its pillasters are Art Nouveau murals.

On the Centre Block's east side, near the Senate foyer and used as a meeting room for senators, is the Salon de la Francophonie, dedicated to Canada's membership in the Organisation internationale de la Francophonie. The space is in a Tudor Gothic Revival style, with oak panelling carved with tracery, exposed wood beams, and textured plaster, as well as bosses carved into flowers, human faces, and gargoyles. A limestone fireplace is flamked by reading nooks with half-vaulted canopies.

It was originally used as a smoking room until, in 1992, the Canadian wing of the Assemblée parlementaire de la Francophonie proposed a commemoration of the country's place in la Francophonie. The following year, the Senate approved granting the room its present name. It is furnished with French emblems, such as fleurs-de-lis and portraits of French monarchs during the time of New France. There are also bronze busts of Samuel de Champlain and Senator Raoul Dandurand. The room is mirrored by the Commonwealth Room, the House of Commons' former smoking room.