Brown, 29, became a free agent when the Washington Redskins released him on March 13.
Middle linebacker became a need position for the Eagles when Jordan Hicks signed with the Arizona Cardinals in free agency. Brown will help bolster a group that includes Nigel Bradham, Kamu Grugier-Hill, Paul Worrilow, Nathan Gerry and L.J. Fort.
Brown would make flashy plays, but the Redskins wanted more consistency, both as a run stopper and in coverage. Though he played with an oblique injury last year, he was relegated to a backup role for the final four games, more for those issues than because he was hurt. After his benching, Brown said he could see the "writing on the wall" with his time in Washington.
Brown, a second-round pick by Tennessee in 2012, made the Pro Bowl two years ago in his only season with Buffalo. The Redskins, wanting to improve their defensive speed, signed Brown the following offseason to a one-year deal. He finished with a team-best 127 tackles, despite missing three games. He was leading the NFL in tackles when he got hurt in Week 14.
They re-signed him last offseason to a three-year deal worth up to $24 million, though the decision was not viewed as a unanimous one throughout the organization. The Oakland Raiders also had expressed interest in Brown last offseason. His release saves $5.75 million in cap space.
Brown also was made expendable when the Redskins claimed Reuben Foster in November. Foster and Brown play the same inside linebacker position in the Redskins' base 3-4 front. Neither one is considered a play-caller, so they would not be an ideal pairing.
Countess was a sixth-round pick by the Eagles in 2016. He has two interceptions.
ESPN's Tim McManus and John Keim contributed to this report.