He has previously played in NPB for the Yomiuri Giants, and in Major League Baseball (MLB) for the Boston Red Sox.

[12] On February 16, 2021, Sawamura officially signed a two-year, $3 million contract with the Boston Red Sox of Major League Baseball (MLB).

[20] Overall during the regular season, Sawamura made 55 relief appearances for Boston, compiling a 5–1 record with 3.06 ERA while striking out 61 batters in 53 innings.

[21] He made three postseason relief appearances, all in the American League Championship Series, allowing a single run in two innings pitched.

After posting a 3.60 ERA in 18 relief appearances, he was optioned to the Triple-A Worcester Red Sox on May 28,[22] but recalled to Boston two days later.

[27][28] On August 10, it was announced that Sawamura had been diagnosed with reversible cerebral vasoconstriction syndrome and spent a short stint in the hospital.

[2] Sawamura gained attention from NBC Sports for singing a rendition of an Alicia Keys song that Eduardo Rodríguez posted on his Instagram account.