Hideki Okajima pitched in the 1994 Koshien National High School Tournament in Japan and was drafted in 1993 by the Yomiuri Giants. There, he became the team's closer in 2000.

He was traded to the Hokkaido Nippon Ham Fighters right before the start of the 2006 season and had a great year with a 2.14 ERA.

Hideki became a free agent after the season and signed a two-year deal with the Red Sox. After he allowed a home run on his very first pitch in the major leagues, he then held opponents scoreless for nearly two months.

At the start of the 2007 season, injuries and poor performances allowed him to step into the role of primary setup man for closer Jonathan Papelbon and he quickly became one of the most dominant relievers in the major leagues.

He was named the American League "Rookie of the Month" for April and pitched a scoreless streak from April to May, the longest by a Red Sox left-hander since Bruce Hurst had done it 20 years prior.

Hideki made the All-Star team but did not play in the game. Following the All-Star break, he continued to pitch solid and reliable relief innings occasionally closing games for the Sox as they held their place at the top of the AL East.

However, the rigors of the long MLB season began to catch up with him in the later stages of the season and he began to struggle. Sox management decided to shut him down and allow him to relieve the physical and mental fatigue of the longer American season.

On his return during the final stages of the regular season, he regained his sharp early season form, pitching scoreless innings to establish himself again as an integral part of the dominant Red Sox bullpen as they headed into the postseason.

Hideki's performance really returned in Game #2 of the 2007 World Series against the Colorado Rockies, in which he pitched 2 1/3 perfect innings in relief of Curt Schilling, striking out four. He was the first Japanese-born pitcher to play in the World Series.

In December 2010, he was non-tendered by the Red Sox, making him a free agent, but re-signed with the team. He started the season in Triple-A Pawtucket but was quickly called up. In May, he was designated for assignment, cleared waivers, and went back to the PawSox.

A free agent at the end of the season, the Yankees signed Hideki to a minor league contract.