Route 31 is a 58.80-mile-long (94.63 km) north–south state highway in the U.S. state of Massachusetts. It runs from Dudley on the Connecticut border to Ashby on the New Hampshire border.

Route 31 begins in Dudley at the Connecticut border, where it is known as Dresser Hill Road. Dresser Hill Road begins in Quinebaug, Connecticut, at Route 197 where it is a short (approximately 220-foot-long (67 m)) local road erroneously signed as Connecticut Route 31 changing to Route 31 at the Massachusetts state line, then proceeding through Dudley and into Charlton, winding over Dresser Hill with several moderately steep grades and some tight corners.

In Charlton, it intersects U.S. Route 20 a few miles east of the I-90 and I-84 interchange. This provides its only connection with an interstate highway. In Charlton, it is known as Masonic Home Road and Brookfield Road. It then passes Lambs Pond.

Route 31 then enters Spencer, where it parallels the Podunk Pike (Route 49) for several miles. As Charlton Road, it passes the Spencer State Forest. In the center of Spencer, it crosses Route 9. It then turns north towards Paxton; this stretch is scenic and hilly. Several warning signs are seen here for sharp corners, warning of a maximum safe speed of 30 miles per hour (48 km/h).

After a couple long, sweeping corners through a forest of pine trees, Route 31 enters Paxton. Paxton Center School comes after a treed-in section. The intersections of Route 31, Route 122, and Route 56 comprise Paxton Center. Route 31 is variously known as West Street and Holden Street in Paxton; it runs east–west in this section. It passes over a hill past Richards Memorial Library, then uses Grove Street for approximately 1⁄4 mile (0.40 km).

In Holden, Route 31 passes Asnebumskit Pond and is bridged over Kendall Reservoir. After the intersection with Route 122A it passes Gale Free Public Library and heads north once again. The road heads into an increasingly rural area as the hills get steeper and longer.

Route 31 then enters Princeton and begins climbing very steeply; it gains about 300 feet (91 m) in approximately 1⁄2 mile (0.80 km). Entering the center of Princeton, it joins Route 62 and turns east to avoid going over Mount Wachusett. The mountain is instead connected by Mountain Road. After separating from Route 62, Route 31 joins Route 140 and becomes Fitchburg Road near Paradise Pond.

It passes through a portion of Leominster State Forest in Westminster before an interchange with a freeway segment of Route 2. After this interchange, Route 31 becomes a city street, having shared roadway with Route 2A and a wrong-way concurrency with Route 12. It then passes through downtown Fitchburg. As it leaves Fitchburg it is called Ashby State Road. In Ashby, it connects with Route 119 before reaching the New Hampshire state border, where it becomes New Hampshire Route 31.

The stretch of Route 31 from Paxton Center to Holden Center is named the Chief Robert J. Mortell Memorial Highway, after Paxton chief of police Robert Mortell, who was killed in the line of duty on February 1, 1994, in the deeply wooded area bordering Route 31.

County	Location	mi	km	Destinations	Notes
Worcester	Dudley	0.0	0.0	To Route 197	Connecticut state line; short connection via Dresser Hill Road
	Charlton	9.3	15.0	US 20 – Sturbridge, Springfield, Worcester, Marlboro
	Spencer	16.8	27.0	Route 9 east – Worcester	Southern terminus of Route 9 concurrency
		16.9	27.2	Route 9 west – Brookfield	Northern terminus of Route 9 concurrency
	Paxton	25.3	40.7	Route 122 / Route 56 south – Barre, Athol, Worcester, Leicester	Southern terminus of Route 56 concurrency
		25.4	40.9	Route 56 north – Rutland	Northern terminus of Route 56 concurrency
	Holden	30.1	48.4	Route 122A – Rutland, Barre, Worcester, Milbury
	Princeton	37.7	60.7	Route 62 west – Hubbardston, Barre	Southern terminus of Route 62 concurrency
		38.0	61.2	Route 62 east – Sterling, Clinton	Northern terminus of Route 62 concurrency
		40.5	65.2	Route 140 south to I-190 – Sterling, Shrewsbury	Southern terminus of Route 140 concurrency
		42.1	67.8	Route 140 north – Westminster, Winchendon	Northern terminus of Route 140 concurrency
	Fitchburg	45.8	73.7	Route 2 – Concord, Boston, Athol, Greenfield	Exit 95 on Route 2
		47.0	75.6	Route 2A west – Westminster, Gardner	Southern terminus of Route 2A concurrency
		48.1	77.4	Route 12 north – Ashburnham, Winchendon	Southern terminus of Route 12 concurrency
		48.9	78.7	Route 12 south – Leominster, Worcester	Northern terminus of Route 12 concurrency with Routes 31/2A/12
		47.0	75.6	Route 2A east – Lunenburg, Ayer	Northern terminus of Route 2A concurrency
Middlesex	Ashby	55.4	89.2	Route 119 west – Ashby, Rindge, NH	Southern terminus of Route 119 concurrency
		55.5	89.3	Route 119 east – Townsend, Groton	Northern terminus of Route 119 concurrency
		58.8	94.6	NH 31 north – Greenville	Continuation into New Hampshire
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Concurrency terminus

• Mass. Rt. 31 endpoint photos