It was the second consecutive monthly honor for Gattis, who also took home the April award. He is the first rookie to win consecutive NL Rookie of the Month Awards since teammate Jason Heyward did so in April and May 2010.
Gattis led all Major League rookies with 16 RBIs in May, while his six homers tied San Diego's Jedd Gyorko for the most in the bigs. Gattis also ranked among Major League rookie leaders in extra-base hits (tied for second), runs (tied for third) and doubles (tied for fifth), while posting a .303/.362/.683 line.
Gattis, 26, capped May by going 3-for-4 in Atlanta's 11-3 win Friday over the Blue Jays.
Gattis' 12 home runs also rank first among NL rookies while ranking fifth in the league.
He also was 4-for-6 as a pinch-hitter, including a two-out, game-tying homer in the bottom of the ninth on May 21 against the Twins.
Cash Kruth is a reporter for MLB.com. Follow him on Twitter at @cashkruth. This story was not subject to the approval of Major League Baseball or its clubs.