The league also said Tuesday the Redskins "assured" the NFL that the team will "re-emphasize with club executives the importance of abiding by the anti-tampering policy and ensure compliance with those rules."
Violation of those rules can result in fines and forfeiting draft picks.
The league said it conducted "an extensive review" of the Haynesworth case.
His former team, the Tennessee Titans, sent information to the NFL indicating the Redskins might have contacted Haynesworth and his agent, Chad Speck, before free agency began Feb. 27.
Haynesworth signed a seven-year, $100 million contract with Washington, with $41 million in guaranteed money.
Spokesmen Robbie Bohren of the Titans and Zack Bolno of the Redskins declined to comment on the NFL's ruling Tuesday.