The deal includes a $1.8 million signing bonus for the 6-foot-4, 247-pound Davis, who was taken in the second round, 48th overall, out of Southern California in April's NFL draft.
"I can say I'm finally a Redskin," Davis said.
Davis said it was important for him to sign before training camp began.
"I didn't want to lose any time, any chance to play or get out there," he said. "I wanted to learn everything. I knew that would be the smartest thing."
Also Saturday, Washington released tight end Tyler Ecker, a 2007 seventh-round draft pick who spent all of last season on injured reserve.