Most Home Runs in a season with multiple teams: Mark McGwire 58, ...

Mark McGwire is the only player to hit at least 50 Home Runs (HR) in a season playing for multiple teams.

>=50 HR, 48 times, ONE for multiple teams: https://stathead.com/tiny/fpfNi

Mark McGwire: 1997 58: Oak, StL

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49 HR, 20 times, none for multiple teams: https://stathead.com/tiny/pOJb9

48 HR, 15 times, none for multiple teams: https://stathead.com/tiny/7Acb0

47 HR, 26 times, none for multiple teams: https://stathead.com/tiny/ruAUA

46 HR, 24 times, none for multiple teams: https://stathead.com/tiny/PHPUd

45 HR, 23 times, ONE for multiple teams: https://stathead.com/tiny/4wdXr

J.D. Martinez: 2017 Ari, Det

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44 HR, 31 times, none for multiple teams: https://stathead.com/tiny/XfvFx

43 HR, 26 times, none for multiple teams: https://stathead.com/tiny/P7wAJ

42 HR, 42 times, none for multiple teams: https://stathead.com/tiny/9LjaZ

41 HR, 49 times, TWO for multiple teams: 

David Justice: 2000 Cle, NYY

Greg Vaughn: 1996 Mil, SDP

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40 HR, 59 times, ONE for multiple teams: https://stathead.com/tiny/tCMjN

Adam Dunn: 2008 Ari, Cin

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I checked homers in the 30s. Technically Mike Piazza hit 32 HR in 1998 playing for three teams: LAD, FLA, NYM. Piazza was parked in FLA on his way from the Dodgers to the Mets: HR 9 + 23 = 32. For FLA he played only 5 games, 18 AB, 0 HR.

No other player hitting 30-39 HR played for more than two teams.