Ephraim Williams Jr. (March 7, 1715 - September 8, 1755) was the benefactor of Williams College, located in northwestern Massachusetts.

Ephraim Jr. was the eldest son of Ephraim Sr. (1691-1754) and Elizabeth Jackson Williams (d.1718). He was born in Newton, Massachusetts, and was raised by his maternal grandparents after his mother died in 1718. In his youth, Ephraim Jr. was a sailor. In 1742, he moved to Stockbridge, Massachusetts, where his parents had relocated, and joined the local militia. He was commissioned Captain in 1745 and put in charge of building and defending Fort Massachuesetts. His family was influential in western Massachusetts; so influential, in fact, that they were often referred to as the "River Gods" (referrencing the Connecticut River, the major waterway in the area). Ephraim Jr. served in both King George's War (1745-1748) and the French and Indian War (1754-1763). It was during the Battle of Lake George that he was shot in the head during an ambush by the French.

Ephraim Jr.'s will left his sizeable estate to support the founding of a free school on his land in western Massachusetts. The school was founded in 1791 and converted to a college in 1793.

Ebenezer Fitch, The first President of Williams College, wrote a biographical sketch of Ephraim Jr. in 1802. He described the College's benefactor as follows: "In his person, he was large and fleshy . . . His address was easy, and his manners pleasing and conciliating. Affable and facetious, he could make himself agreeable in all companies; and was very generally esteemed, respected, and beloved."

There are no known portraits of Ephraim Jr.