Mark McGwire hit **70** home runs in one season.
It was in 1998 when Mark McGwire hit 70 home runs, breaking the single-season home run record of 61 set by Roger Maris in 1961. McGwire's record-breaking home run came on September 8, 1998, off of Steve Trachsel of the Chicago Cubs.
McGwire's record-breaking season was part of a home run race with Sammy Sosa of the Chicago Cubs. Sosa finished the season with 66 home runs, which was the second-most home runs in a single season.
McGwire's record-breaking season was also part of a larger era of increased home run hitting in Major League Baseball. This era is often referred to as the "Steroid Era", as many players were suspected of using performance-enhancing drugs.
McGwire has admitted to using steroids during his career, but he has said that he did not use them during the 1998 season.