In 28 games last month, Brown led the National League with 12 home runs and 25 RBI, tying Miguel Cabrera for the Major League lead in homers while ranking fourth overall in RBI.
Brown also had 89 total bases which was good for third in the Majors.
Brown slugged .688 during the month of May, the second-highest mark in the N.L., while posting a .303 (33-for-109) batting average.
The left-handed-hitting slugger became the first player in Major League history to hit at least 10 home runs in a calendar month without a walk.
He is the first Phillie to eclipse 12 homers in a single month since Ryan Howard clubbed 14 in August 2006 en route to claiming the N.L. MVP award.
Brown capped his stellar May with his 12th multi-hit effort of the month in Friday's series-opener with the Milwaukee Brewers at Citizens Bank Park, going 2-for-4 with two homers and 4 RBI in an 8-5 loss.
It was his second multi-homer performance in three days after clubbing a pair in Philadelphia's May 29th victory over Boston.
In that outing, Brown's second homer of the day, a solo shot in the eighth inning, proved to be the difference in a 4-3 win.
It was one of three game-winning RBI he had for the month. Brown homered, doubled and drove in two runs in the middle game of a three-game set in Washington on May 25th.
In that contest, Philadelphia's 20th round pick in the 2006 First-Year Player Draft joined catcher Erik Kratz in clubbing back-to-back solo shots in the top of the second inning to give the Phillies an early 2-0 lead.
Brown's RBI double in the eighth inning provided insurance to the Phillies bullpen as they shut the door on Washington for a 5-3 win.
This is the first career monthly award for the 25-year-old, who is the first Phillies player to win Player of the Month honors since Ryan Howard was voted the winner for August 2009.