The 31st Massachusetts was an infantry regiment that served in the Union Army during the American Civil War.

The regiment was attached to 2nd Brigade, Department of the Gulf, to October 1862.

3rd Brigade, 3rd Division, XIX Corps, Department of the Gulf, to July 1862.

2nd Brigade, 1st Division, XIX Corps, Department of the Gulf, to December 1863.

4th Brigade, Cavalry Division, Department of the Gulf, to June 1864.

1st Brigade, Cavalry Division, Department of the Gulf, to October 1864.

Moved to Boston February 19, then sailed on the steamer Mississippi for Ship Island, Miss.

Detained at Hilton Head, S.C., repairing vessel, March 1–13.

Operations against Forts St. Phillip and Jackson, Mississippi River, April 18–28, 1862.

Skirmish at Bayou Bontecou November 21, 1862, and Deserted Station December 10.

Moved to Baton Rouge September 1 and duty there until December 9.

Moved to New Orleans December 9 and there converted into a mounted infantry regiment.

March to Berwick Bay and Brashear City February 29-March 9.

Bayou de Paul, Carroll's Mill and Sabine Cross Roads April 8.

Ordered to Donaldsonville November 27, and operated against guerrillas until February 1865.

Expedition from Plaquemine to the Park January 26 to February 4 (detachment).