 In April, he was elected as March's MVP (Player of the Month) in the J-League.On 30 May 2015, Muto joined Bundesliga club Mainz 05 on a four-year contract until 2019.On 31 October 2015, he scored a hat-trick for the first time in his career and also became the second Japanese player after Naohiro Takahara to score a hat-trick in the Bundesliga, in his team's 3–3 draw against FC Augsburg.