The Crimson Tide (14-1) became the second team in college football's poll era, dating back to 1936, to win four titles in seven seasons.
Alabama joins Notre Dame, which won four titles from 1943-49. For Alabama coach Nick Saban, it is his fifth national championship, leaving him only one short of former Tide coach Bear Bryant for the most all-time.
Alabama hit Clemson (15-0) with a barrage of big plays, including four touchdowns of at least 50 yards, and used a surprising onside kick early in the fourth quarter to seize control.
Watson gave the Tide all it could handle, throwing for 405 yards and four touchdowns.