In 2014, Romero made the Mariners' Opening Day roster,[11] and had his major league debut on April 1.[12] He struggled, batting .192 in 73 games.[13] He began the 2015 season in Tacoma, playing in 116 games for the team, and also went 4-for-21 in 24 plate appearances for the Mariners. He began the 2016 season in Tacoma, and went 4-for-17 in 19 plate appearances for the Mariners that year.[14][15]
On November 19, 2016, the Mariners released Romero to allow him to pursue an opportunity to play in Japan.[16]
Orix Buffaloes
He signed with the Orix Buffaloes of Nippon Professional Baseball's Pacific League on November 24, 2016.[17] In 2017, Romero slashed .274/.330/.508 with 26 home runs and 66 RBI. He signed a three-year contract extension with the Buffaloes on August 4, 2017, earning $2.5 million per season.[18] In 2018, Romero batted .237/.313/.451 with 25 home runs and 63 RBI. In 2019, Romero played in 81 games for Orix, batting .305/.363/.539 with 18 home runs and 63 RBI. On December 2, 2019, he became a free agent.[19]
Tohoku Rakuten Golden Eagles
On January 27, 2020, Romero signed with the Rakuten Golden Eagles of Nippon Professional Baseball(NPB).[20] On February 15, 2020, he held press conference.[21] On the year, Romero slashed .272/.354/.539 with 24 home runs and 63 RBI for Rakuten. On December 2, he became a free agent.[22]
Orix Buffaloes (second stint)
On January 8, 2021, Romero signed a one-year contract to return to the Orix Buffaloes of NPB.[23] Due to the influence of COVID-19, he was in a situation where his family could not come to Japan and lived away from his family. The Buffaloes acknowledged his desire to prioritize his family and he decided to leave the team on August 3.[24][25]
Los Angeles Dodgers
On February 12, 2022, Romero signed a minor league contract with the Los Angeles Dodgers.[26] After beginning the season with the Triple-A Oklahoma City Dodgers he was added to the major league roster on June 22.[27] Romero was designated for assignment on June 25, without appearing in a game.[28] He cleared waivers and was sent outright to Triple-A Oklahoma City on June 27.[29] Romero played in 30 games for Oklahoma City, hitting .265/.353/.431 with three home runs and 22 RBI[30] before he was placed on the injured list on July 9 and stayed there the rest of the season.[31] He elected free agency following the season on November 10.[32]
Diablos Rojos del México
On March 6, 2023, Romero signed with the Diablos Rojos del México of the Mexican League.[33] In 36 games for México, he slashed .322/.403/.568 with eight home runs and 28 RBI.
Romero was placed on the reserve list on March 23, 2024, and did not appear in a game for the team. The Diablos would go on to win the Serie del Rey.[34] Romero retired from professional baseball on November 19.[35]
Personal life
Stefen's younger brother, Santiago, was a teammate on their high school baseball team.[1]