Eovaldi's deal is worth $68 million and will pay him an even $17 million each of his four seasons, sources told Jeff Passan of Yahoo Sports.
"We're very happy to have Nathan back with us," Red Sox president of baseball operations Dave Dombrowski said in a statement, according to MLB.com's Ian Browne. "He did a tremendous job for us last season, playing a significant role in helping us win the division and the World Series. His performance in the postseason was outstanding, both as a starting pitcher and as a reliever."
Eovaldi, 28, posted a 3.33 ERA and 2.88 FIP over 54 innings for Boston down the stretch before playing an integral role in the team's championship victory. Over 22 1/3 postseason innings, he allowed four earned runs on 15 hits and three walks. His playoff performance included a six-inning relief outing during the 18-inning marathon in Game 3 of the World Series.
Over seven seasons in the majors with five different teams, Eovaldi's thrown more than 160 innings just once. He missed all of 2017 after undergoing the second Tommy John surgery of his career.
Eovaldi rejoins a rotation that features Chris Sale - who will become a free agent following the 2019 season - David Price, Rick Porcello, and Eduardo Rodriguez. The Red Sox were one of two teams to exceed the luxury-tax threshold in 2018 and are set to blow past it once again after re-signing Eovaldi as well as World Series MVP Steve Pearce.