Arenado - the slick-fielding Colorado Rockies third baseman - erupted for a major-league best seven long balls while posting a .391/.440/.1.348 slash line in six games to secure the National League honor.
The 24-year-old's power surge includes eight homers over his last seven games and a club-record 12 straight contests with at least a hit or run scored. Arenado's record-breaking week was capped by a pair of homers Sunday to increase his season total to 24 - the most by a Rockies player before July 1 since 2001.
Gardner, meanwhile, continued his offensive tear by batting .500/.581/.885 with two homers and four doubles to earn the American League award. The New York Yankees outfielder recorded multi-hit efforts in four of his six games last week.