Kerrigan, 32, was a first-round pick by Washington in 2011 and played the first 10 years of his NFL career for Washington, racking up 95.5 sacks and 147 quarterback hits.
He started just one game in 2020 and played 38% of the team's defensive snaps -- by far the lowest of his career -- but still had 5.5 sacks and five tackles for loss.
Kerrigan has 13.5 career sacks against Philadelphia, tied (Dallas Cowboys) for his most sacks against any one team.
"I know I probably wasn't your favorite player over the past decade, but Eagles fans I'm fired up to be playing for you guys now! LFG!!!" Kerrigan wrote on social media.
Kerrigan joins a defensive end rotation in Philadelphia that includes Brandon Graham, Derek Barnett and Josh Sweat. He'll be playing for new defensive coordinator Jonathan Gannon, who is expected to ask his linemen to read and react more, in contrast to former DC Jim Schwartz's constant attack style.
"They've got Brandon Graham and Derek Barnett, guys who are entrenched and are bona fide studs in this league," Kerrigan said of the Eagles. "But I know they rotate their guys a lot. Whether I'm a starter or a reserve, I still imagine I'll get a lot of playing time. That was a big thing for me."
Washington had left the door open for a possible return, according to a team source. But it also has starting ends Montez Sweat and Chase Young, and it drafted two more in William Bradley-King and Shaka Toney.
Washington also has two other young ends in James Smith-Williams, a 2020 seventh-round pick, and Casey Toohill. The team wanted to see how its young ends had developed before exploring a legitimate reunion with Kerrigan.
In January, Kerrigan told ESPN, "I always envisioned playing my entire career here."
But he said by season's end -- and as a pending free agent -- he had grown used to the thought of playing elsewhere. He remained confident in his ability.
Kerrigan said the chance to play Washington twice did not factor into his signing with the Eagles.
"No, not at all," he said. "I'm not like, 'Oh, I've got to get revenge.' Get revenge on Washington? The place has been so good to me. No, that wasn't a thought at all."
ESPN's John Keim contributed to this report.