The Eagles received Gardner-Johnson and the Saints' 2025 seventh-round draft pick for a 2023 fifth-round draft pick and the latter of their two sixth-round draft picks in 2024.
Gardner-Johnson, who has experience both as a slot corner and safety, is expected to play safety in Philadelphia.
The Eagles created an opening at starting safety before the trade by releasing veteran Anthony Harris, a source told ESPN's Adam Schefter. Gardner-Johnson will be paired with Marcus Epps in the back end.
Gardner-Johnson is in the last year of his rookie deal and was seeking a new contract. He did not fully participate in practice during the beginning of camp, sitting out of team drills and at times just doing walk-throughs, effectively holding in while asking for a new deal.
He returned to full participation on Aug. 8 and Allen said at the time that would be the expectation going forward. He was a full participant after that.
Gardner-Johnson had said this summer that he thought he was the best nickel cornerback in the NFL.
Gardner-Johnson, 24, was selected in the fourth round of the 2019 draft by the Saints. He posted a career-high three interceptions last season and added seven passes defensed, two sacks and four tackles for loss. He has 28 passes defensed over three seasons.
Safety was regarded as the thinnest position on an otherwise sound Eagles roster. They signed former San Francisco 49ers safety Jaquiski Tartt to a one-year deal in June, but he had a quiet summer overall and has reportedly been released.
Gardner-Johnson has the range and position flexibility defensive coordinator Jonathan Gannon looks for in defensive backs.
Harris, 30, played six seasons with the Minnesota Vikings before signing with the Eagles in free agency in 2021. He had 72 tackles, an interception and three passes defensed over 14 games last season, missing time with multiple injuries.
ESPN's Katherine Terrell contributed to this report.