"We're very happy to have Nathan back with us," Red Sox president Dave Dombrowski said in announcing the deal Thursday. "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."
The deal is worth $68 million, with the right-hander getting $17 million in each of the four seasons, according to multiple reports.
The Red Sox will introduce Eovaldi on Monday at the winter meetings in Las Vegas.
Eovaldi, 28, emerged as a star in the postseason, dominating both as a starter and late-inning reliever.
He ranked fourth in the American League in walks per nine innings (1.62), and sixth in strikeout-to-walk ratio (5.05) last season. He held right-handed hitters to a .252 on-base percentage with 54 strikeouts against only five walks, and did not allow a home run to any of his final 194 righties faced, including in the postseason.
Left-handed batters went 4-for-53 with one extra-base hit and a .329 OPS against Eovaldi from Sept. 1 through the end of the World Series.
Eovaldi has had two Tommy John surgeries, most recently in 2016.
After being acquired from the Tampa Bay Rays on July 25, Eovaldi went 3-3 with a 3.33 ERA in 11 starts and one long-relief appearance. He found his groove in September, with a 1.35 ERA in five appearances that gave a hint of what was to come in the postseason.
In 22⅓ postseason innings, Eovaldi allowed only four earned runs, throwing 100 mph fastballs that were unhittable.
The right-hander handcuffed the New York Yankees in Game 3 of the American League Division Series, allowing one run in seven innings of a rout at Yankee Stadium. He stifled the Houston Astros in Game 3 of the AL Championship Series, going six innings and getting the win. He also made three eighth-inning relief appearances that required him to record 10 outs against the Astros and Los Angeles Dodgers.
Eovaldi became a folk hero in Boston with a six-inning stint in the 18-inning loss in Game 3 of the World Series in Los Angeles. Coming on in the 12th inning, Eovaldi threw 97 pitches in relief. Though he got the loss, teammates and coaches called the outing inspirational, citing his stamina and willingness to give his all.
"One of the best performances in the history of the World Series," Red Sox manager Alex Cora said. "What Nate did, that was amazing."
He was released by the Yankees in 2017 and signed a one-year, $2 million deal with the Rays for 2018. He made his return from surgery May 30 and made 10 starts for Tampa Bay, going 3-4 with a 4.26 ERA.
Eovaldi has a 44-53 record and 4.16 ERA since entering the majors in 2011, in stints with the Dodgers and Miami Marlins as well as the Yankees, Rays and Red Sox.