Brian Linder from IGN felt that the track was lighter compared to the other songs on The Fame Monster, and complimented the line "We French kissed on a subway train / He tore my clothes right off / He ate my heart and then he ate my brain", calling it a lyrical gem. He also added that "Monster" was a "dance floor riot". Bill Lamb from About.com listed it as one of the top tracks from The Fame Monster, and felt that "'Monster' with the 'He ate my heart' catch line is pure electronic dance pop heaven in sound." Jaime Gill from Yahoo! felt that "'Monster' is a squirmy little beast that wriggles into your brain slowly and is almost impossible to remove." Monica Herrera from Billboard called the song "80s adoring."

In the United Kingdom, "Monster" debuted on the UK Singles Chart at sixty-eight, on December 12, 2009, but slipped off the chart the next week. On August 16, 2010, the song debuted at number thirty on the New Zealand Singles Chart due to digital downloads and radio airplay, and later peaked at number twenty-nine. The song was present on the chart for seven weeks. In Hungary, it debuted on the 'Mahasz Single Top 10' lista 'chart at number six on November 23, 2009, but fell off the next week. "Monster" debuted and peaked on the Australian ARIA Singles Chart at number eighty on the issue dated November 30, 2009. The song debuted on the United States Billboard Hot Dance Club Songs at number forty-nine on September 18, 2010, before moving to its highest position of number twenty-nine on October 9, 2010, where it remained for another week. It fell off the chart after eight weeks. On the Latin Pop Airplay chart, the song was initially seen at number thirty-two, and later peaked at number twenty-two. "Monster" has spent fourteen weeks on the chart. According to Nielsen Soundscan, the song has sold 207,000 digital downloads in the US.