Samuel Adams was a Congregationalist.
He was identified as a Congregationalist by The Congregationalist Library. (Source: Ian Dorion, "Table of the Religious Affiliations of American Founders", 1997).
From: Robert G. Ferris (editor), Signers of the Declaration: Historic Places Commemorating the Signing of the Declaration of Independence, published by the United States Department of the Interior, National Park Service: Washington, D.C. (revised edition 1975), page 36-38:
"Firebrand of the Revolution," Samuel Adams probably more than almost any other individual instigated and organized colonia resistance to the Crown. A talented polemicist and agitator-propagandist who relied more on his facile pen than the podium in behind-the-scenes manipulation of men and events, he religiously believed in the righteousness of his political causes, to which he persistently tried to convert others. He failed in business, neglected his family, gained a reputation as an eccentric, and demonstrated as much indifference to his own welfare as he did solicitousness for that of the public. His second cousin John Adams, more of a statesman, eclipsed him in the Continental Congress, though Samuel signed both the Declaration and the Articles of Confederation. In his later years, the Commonwealth of Massachusetts bestowed on him many high offices, capped with the governorship.
...At the age of 42, unable to support a new wife and two children from his first marriage and residing in his rundown birthplace, he was destitute and besieged by creditors. He subsisted mainly on gifts and donationations from loyal friends and neighbors.
Adams was a failure by most standards, but he had long before found the only meaningful "occupation" he ever pursued. For almost two decades he had been active in local political clubs, where he earned a reputation as a writer and emerged as leader of the "popular" party that opposed the powerful aristocracy controlling the Massachusetts government. [This was prior to 1774.]
...He refused to attend the Constitutional Convention of 1787 because of his objection to a stronger National Government, and the following year unenthusiastically took part in the Massachusetts ratifying convention. A lifetime of public service culminated in his election as Lieutnant Governor (1789-93), interim Governor in the latter year upon Hancock's death, and Governor (1794-97). Still living in "honest poverty," he died at Boston in 1803 at the age of 81 and was buried in the Old Granary Burying Ground.
Note that numerous sources and authoritative references have been consulted in order to ascertain the religious affiliation of the American Founding Fathers. Note that the excerpts and references mentioned on this page are not the only references used in order to identify this person's religious affiliation.