The Ohio Statehouse is the state capitol building and seat of government for the U.S. state of Ohio. The Greek Revival building is located on Capitol Square in Downtown Columbus. The capitol houses the Ohio General Assembly, consisting of the House of Representatives and the Senate. It also contains the ceremonial offices of the governor, lieutenant governor, state treasurer, and state auditor. Built between 1839 and 1861, it is one of the oldest working statehouses in the United States. The statehouse grounds include two other buildings, the Judiciary Annex or Senate Building, and the Atrium; the three are collectively referred to as the Ohio Statehouse into the present day.

The statehouse's prominent architecture has earned it several landmark designations, including as a National Historic Landmark. The building sees about 500,000 visitors per year.

Chillicothe was Ohio's first state capital, from 1803. Due to political fighting among state leaders, the Ohio General Assembly temporarily moved the capital to Zanesville in 1810. Legislation enacted in that year provided for the selection of a new state capital "not more than 40 miles (64 km) from what may be deemed the common center of the state." In 1812, the General Assembly restored Chillicothe as the temporary state capital until the new capitol could be built.

State leaders faced ongoing pressure to make the capital city more accessible by moving it closer to the center of the state. In response, the legislature appointed a committee to evaluate potential options for the new capital city. Four prominent Franklinton area land owners proposed a largely empty tract of land across the river from Franklinton. The land owners offered to donate two 10-acre (4.0 ha) parcels of land to the state and proposed spending up to $50,000 to build structures and make other improvements to the area.

On February 14, 1812, the General Assembly created a new capital city on the "High Banks opposite Franklinton at the Forks of the Scioto most known as Wolf's Ridge". Approximately a week later, after intense debate, the legislature selected Columbus as the name of the new town. In the following months the new town was surveyed and laid out in rectangular grid. The layout set aside two 10-acre parcels of land, one for a statehouse, at the location of the present day Statehouse, and the other parcel would become the site of the Ohio Penitentiary. Public sale of town lots for the new city began in June 1812. In 1816, the General Assembly met in Columbus for the first time in a brick building on the corner of High and State Streets.

The land for the eventual permanent statehouse was planned during the initial platting of the city. 10 acres were selected, and Jarvis Pike was hired to clear the land and enclose it in a stake-and-rider fence. In return, Pike was allowed to grow corn and wheat on the plot for years.

In 1838, Ohio's government announced a competition to select the design for a new Statehouse. This strategy was not unusual at the time, as important public buildings such as the U.S. Capitol had resulted from similar contests. From about fifty entries, three winners were selected: first prize was awarded to Henry Walter of Cincinnati, the second to Martin Thompson of New York, and the third to painter Thomas Cole, also of New York. However, the organizing commission responsible for choosing the winners was unable to agree on a final design for construction.

When the cornerstone was laid on July 4, 1839, the commission was still without a final design. Consultation with New York architect Alexander Jackson Davis resulted in a composite design that merged some key features of the three winning entries, but it was rejected as being too expensive. Henry Walter, the first-place winner of the design contest, was chosen to supervise construction of the new capitol and he began working on another composite design that was based largely on the design of third-place winner, Thomas Cole. Cole had a personal friendship with one of the commissioners, a man named William A. Adams who was from Steubenville. Cole's nephew, William Henry Bayless, a Steubenville native, was apprenticed in the office of Alexander Jackson Davis.

Work on the building's foundation and lower level had only just begun when the Statehouse project encountered the first of many difficulties. The legislation that made Columbus the official capital city of Ohio was set to expire. While various factions within the government engaged in debate over relocating the capitol to another city, construction of the Statehouse was stopped. Open excavations were refilled with earth, and Capitol Square became open pasture for livestock.

The Statehouse remained neglected until February, 1848, when William Russell West and J.O. Sawyer of Cincinnati were appointed architects and general supervisors of the project. By May of that same year construction had resumed. A cholera epidemic began in Columbus, prompting widespread flight to the countryside by residents. Once the epidemic subsided, work on the Statehouse continued, interrupted only by intermissions during the harsh Ohio winters. Comparing plans of the various architects it is apparent that perhaps the most striking change and one that has endured in the finished building, was the fact that West and Sawyer eliminated the rounded dome that all previous designers had suggested for the building, instead replacing it with a low conical roof.

Fire consumed the old two-story capitol building in 1852, which created a new urgency to complete the Statehouse project as government offices were forced to relocate to various buildings around Columbus. While some suspected arson, the exact cause of the fire remains a mystery to this day. With the Statehouse exterior nearing completion by 1854, Columbus architect Nathan Kelley was hired to supervise the design and construction of the building's interiors. One of his major tasks would be to provide a system for heating and ventilation in the building, which had not been considered previously. An innovative steam heating system was constructed, with warmed air moved through the building in what Kelley called "air sewers"—small passages made of bricks that linked the various floors of the building. With great fanfare, the new Statehouse opened to the public on January 7, 1857, and soon thereafter the Ohio General Assembly convened in their new chambers. Most of the building was completed, with the notable exception of the rotunda. Although actual work on the building was proceeding smoothly, government officials were deeply dissatisfied with Kelley, citing problems with his working methods and aesthetic choices.

Isaiah Rogers, a well known architect based in Cincinnati at the time, was recruited to supervise the final stages of the Statehouse's construction. During his tenure, Rogers oversaw completion of the building's interior and coordinated work on the distinctive rotunda and its enclosing cupola. One of the building's most distinctive exterior features is the low, conical roof atop the cupola, positioned where many viewers expect to see a dome. In the long span between beginning construction on the Statehouse and its completion, the "finished" design changed many times and various proposals included a round dome atop the building. Rogers, in deciding not to use a dome, was actually reverting to a design scheme by architects West and Sawyer. In the end, the building featured a low conical roof that some critics would deride as "a Chinese hat." Construction of Capitol Square, including its buildings, grounds, and landscaping, was finally completed in 1861.

As the function of state government changed and expanded, changes and expansions occurred at the Ohio Statehouse. Originally, the building was the main location for all aspects of state government. As more offices and work rooms were required, large spaces would be subdivided into smaller areas. The most prominent example was the conversion to offices of the four open courts that occupied areas on the interior of the building. These open areas were from top to bottom of the structure and were intended to admit light and fresh air to the inner reaches of the building. The advent of electric lighting coupled with the need for space meant that levels of offices would come to occupy these large open areas. The fifty-four rooms the building originally held increased to 317 rooms by 1989.

In 1901, facing significant crowding several changes were considered including a completely new statehouse designed to more closely resemble the national Capitol building in Washington. Cost prohibitive, it was decided that the Ohio Supreme Court would move to a separate building on the east side of Capitol Square, and to give the court the prestige of its own building. The new building, named the Judiciary Annex, was constructed of the same Columbus limestone as the Statehouse. Neoclassical on the exterior, the interior spaces, especially the grand central staircase are Beaux Arts in style. The building was the work of Cincinnati architect Samuel Hannaford, and was completed in two years at the cost of $375,000. By comparison, the Statehouse itself took 22 years from start to finish and cost approximately $1.3 million. The difference in cost and time to completion indicates differences in how the buildings were constructed. The Statehouse is more akin to a castle or cathedral, with thick load-bearing walls of stone. The Annex was a modern building with a metal girder skeleton and was planned for the use of electric light. The building retains many of its original light fixtures, while those in the Statehouse are reproductions of the gas lights that were removed to make room for electrical appliances in the 1890s.

Murals painted by William Mark Young for the Ohio State Exhibit at the 1933 A Century of Progress International Exposition, (also known as the Chicago World's Fair) were subsequently moved to the building.

The two buildings became crowded, and decay from heavy usage and inadequate maintenance was evident. Both buildings survived despite proposals made to either demolish one or both buildings, or remodel them substantially. In 1989, a massive project commenced to restore the buildings to the splendor their original builders envisioned as well as make them useful and practical government buildings. Original furniture was sought for return to the building when possible and modern reproductions of long gone items such as carpets and light fixtures were created. The large scale light fixtures in the House and Senate were based in large part on surviving period fixtures in the Vermont State House, which was being restored at the same time. The restoration project also resulted in the addition of a third building to Capitol Square. The Atrium, which connected the Statehouse with the Judiciary Annex, was completed in 1993. After the renovation, the Judiciary Annex became the Senate Building.