On March 10, 2018, Moustakas re-signed with the Royals for one year and $5.5 million, plus a mutual option for 2019.
On July 28, 2018, Moustakas was traded to the Milwaukee Brewers in exchange for Brett Phillips and Jorge López. With the Brewers, he batted .256/.326/.441.
On October 4, 2018, Moustakas hit a walk-off single in the 10th inning of NLDS Game 1 vs the Colorado Rockies, giving the Brewers a 3-2 win and a 1-0 series lead. The Brewers later swept the series in three games, but would proceed to lose the NLCS to the Los Angeles Dodgers 4 games to 3. On October 30, 2018 Moustakas declined his half of a mutual option for 2019 and became a free agent.