In 28 games in May, the left-handed-hitting slugger hit 12 homers, had 33 RBIs, scored 23 runs, slugged .739 and mounted 19 extra-base hits to lead the NL in each of those categories.
Bruce is batting .292 this season, with 17 homers (best in the NL), 46 RBIs (tied for first in baseball) and 39 runs scored for the Reds, who sit four games behind the Cardinals in the NL Central as of Thursday afternoon.
His home run and RBI totals were the highest in a calendar month for a Reds batter since Adam Dunn's 12 homers in July 2008 and Greg Vaughn's 33 RBIs in September 1999.
The right fielder also tallied 38 hits and hit .342 (38-for-111) in the month.
It is Bruce's first career monthly award. He received his second career National League Player of the Week honor this month, during the week of May 23-29.