The 3rd Battalion Massachusetts Rifles was an infantry unit created at the start of the American Civil War and activated for federal service in the Union army for a term of ninety days.

[2] The organization of the battalion began on April 19, 1861 in response to Abraham Lincoln's call for 75,000 volunteer troops at the start of the war.

On April 19, 1861, one week after Confederate forces fired on Fort Sumter, Major Devens ordered his battalion to assemble in Worcester to prepare for active service.

The same day, Devens received orders from Governor John Andrew to depart for Washington, D.C.[5] The "Worcester City Guards" made up Company A of the battalion.

The circumstances leading to its creation arose from local political tensions arising out of the election of 1840 as the older Worcester Light Infantry company split along party lines.

[7] The "Emmett Guards" of Worcester formed Company C. The unit was organized in 1858 and commanded by Captain Michael McConville.

[4] At the close of the 90-day term, the battalion was asked by Major General John Adams Dix to remain at Fort McHenry for another two weeks due to a shortage of garrison troops.

Though twelve men decided to return home at the end of their term, the great majority of the battalion remained another two weeks at Fort McHenry as requested.