A baseball signed by World Series Champion and Boston Red Sox pitcher Nathan Eovaldi

In the 2018 playoffs, Eovaldi pitched seven innings in Game 3 of the Division Series against the New York Yankees, which the Red Sox won 16–1, giving the Yankees their most lopsided loss in postseason history. In Game 3 of the League Championship Series against the Houston Astros, Eovaldi pitched six innings, allowing just two runs, as the Red Sox won 8–2. Eovaldi also pitched ​1 1⁄3 innings of relief in Game 5 as Boston clinched the series win.

In the 2018 World Series against the Los Angeles Dodgers, Eovaldi pitched shutout eighth innings in Games 1 and 2. Although he was originally slated to start Game 4, Eovaldi ended up the final reliever in the 18-inning marathon Game 3, the longest game in World Series history. Eovaldi threw 97 pitches over six innings of relief before giving up a walk-off home run to Max Muncy, which made him the losing pitcher. Eovaldi allowed three hits and one earned run, while throwing 36 more pitches than starter Rick Porcello. Eovaldi's 97 pitches set the record for the most in a World Series game by a reliever, and he became the first reliever to throw 6+ innings in a World Series game since Rick Rhoden did so in 1977. In the 13th inning, Boston had taken the lead but an error by Ian Kinsler allowed Los Angeles to tie the score, for which Kinsler later apologized to Eovaldi. Eovaldi received a standing ovation in Boston's clubhouse after the game. The Red Sox went on to win the next two games, winning the World Series in five games.