The left-hander finished the previous month with a 5-0 record and 0.45 earned run average, allowing only two earned runs in 39 2/3 innings pitched.
He held the opposition scoreless in four of those five outings, surrendering the only scores to Arizona in a 9-2 victory on August 17.
With that stingy effort, Lee became the first pitcher in baseball history to record two months in one season with at least five wins, no losses and an ERA below 0.50 -- after doing the same in June to take home top pitching honors.
For the 33-year-old Arkansas native, it is his fifth career monthly award, winning in April and August of 2008 with Cleveland and June 2010 while pitching for Seattle. He also became the first Phillies pitcher to win twice in one season.