Masopust was born in the town of Božejov, near Pelhřimov.[1] He began his football career there, before moving to Kamenice nad Lipou and then to Jihlava in 2004.[1] In 2012, Jihlava manager František Komňacký promoted him to the senior team and he went on to make his debut for the club against Slavia Prague that year. The match ended in a 3–3, to which Masopust contributed two assists.[1]
In December 2014, Masopust reached an agreement to join Jablonec on 1 January 2015, on a contract running until 1 July 2018. He made his debut for the club against 1. FC Slovácko on 20 February 2015, coming on as a 75th minute substitute for Nermin Crnkić.
Slavia Prague
On 18 December 2018, Slavia Prague announced the signing of Masopust from Jablonec on a three-and-a-half-year contract.[1] On 22 May 2019, Masopust scored Slavia's second goal in their 2–0 defeat of Baník Ostrava in the 2019 Czech Cup Final.[3][4] On 20 August 2019, Masopust scored in Slavia Prague's UEFA Champions League first leg play-off win against Romanian side CFR Cluj.[5] Slavia would win the second leg as well to qualify for the group stage of Europe's premier knock-out competition for just the second time in their history.[6][7]
Masopust started for Slavia in the final of the Czech Cup on 20 May 2021, but was replaced by 19-year-old teammate Abdallah Sima in the 60th minute.[8] Sima would go on to score the only goal of the game against Viktoria Plzeň as Slavia won the league and cup double.[8] In the final match of the league season on 29 May 2021, Slavia defeated Dynamo České Budějovice to clinch an undefeated record in the league. It was the first time a Czech club reached this milestone since rivals Sparta Prague did so in 2009–10.[9]
On 25 May 2021, Masopust was included in the final 26-man squad for the postponed UEFA Euro 2020 tournament.[11] In the final warm-up match before the tournament on 8 June, Masopust scored as the Czechs defeated Albania 3–1.[12]
^Two appearances in UEFA Champions League, twelve appearances in UEFA Europa League
^Two appearances and one goal in UEFA Champions League, one appearance and one goal in UEFA Europa League, seven appearances in UEFA Europa Conference League