The deal is worth $22.5 million and includes $13.3 million guaranteed, sources told ESPN's Adam Schefter.
Maddox, 25, has 31 tackles and one interception this season.
A fourth-round pick out of Pitt in the 2018 draft, Maddox has 170 tackles and three interceptions in his career.
The move comes a day after Eagles tight end Dallas Goedert agreed to a four-year contract extension that runs through 2025. Goedert's deal is worth $59 million and includes $35.7 million guaranteed, a source told Schefter.