The financial terms of the five-year deal, with the option for a sixth, were not available. The Rams report to training camp at Concordia University in Mequon, Wis., on Thursday.
The Rams anticipate the 279-pound Long starting at right end to beef up an anemic pass rush.
Long's signing means the top three picks have agreed to contracts before training camp. The Dolphins signed first overall pick Jake Long, an offensive tackle from Michigan, before the draft and quarterback Matt Ryan has signed with the Falcons.
The Rams have signed half of their eight picks.
Long is the son of Hall of Fame defensive end Howie Long.
John Clayton covers the NFL for ESPN.com. Information from the Associated Press was used in this report.