These are the rosters of all participating teams at the men's water polo tournament at the 2020 Summer Olympics in Tokyo. The twelve national teams were required to submit squads of 12 players. Additionally, teams could name one alternate player. If a player on the submitted squad list suffered an injury or illness, that player would be able to be replaced by the player in the alternate list. On 3 July 2021, the International Olympic Committee (IOC) confirmed that there was a change for the 2020 Summer Olympics, allowing all 13 water polo players named to be available on the roster, with 12 being named for each match. This change was implemented due to the challenges of the COVID-19 pandemic. As of 2 August 2021, all players competed in the men's tournament.
<section begin=GRE /> Greece's final squad was announced on 3 July 2021.
<section begin=GREWPT />Head coach: Thodoris Vlachos
<small>Note: Age as of 23 July 2021<br />Source: Greece Men | Tokyo 2020 Olympics </small><section end=GREWPT /><section end=GRE />
<section begin=HUN /> Hungary's final squad was announced on 29 June 2021.
<section begin=HUNWPT />Head coach: Tamás Märcz
<small>Note: Age as of 23 July 2021<br />Source: Hungary Men | Tokyo 2020 Olympics </small><section end=HUNWPT /><section end=HUN />
<section begin=ITA /> Italy's final squad was announced on 2 July 2021.
<section begin=ITAWPT />Head coach: Sandro Campagna
<small>Note: Age as of 23 July 2021<br />Source: Italy Men | Tokyo 2020 Olympics </small><section end=ITAWPT /><section end=ITA />
<section begin=JPN /> Japan's final squad was announced on 19 May 2021.
<section begin=JPNWPT />Head coach: Yoji Omoto
<small>Note: Age as of 23 July 2021<br />Source: Japan Men | Tokyo 2020 Olympics </small><section end=JPNWPT /><section end=JPN />
<section begin=RSA /> South Africa's squad was announced on 24 June 2021. Roarke Olver was replaced by Timothy Rezelman.
<section begin=RSAWPT />Head coach: Paul Martin
<small>Note: Age as of 23 July 2021<br />Source: South Africa Men | Tokyo 2020 Olympics </small><section end=RSAWPT /><section end=RSA />
<section begin=USA /> The United States' final squad was announced on 2 July 2021.
<section begin=USAWPT />Head coach: Dejan UdoviÃÂiÃÂ
<small>Note: Age as of 23 July 2021<br />Source: United States Men | Tokyo 2020 Olympics </small><section end=USAWPT /><section end=USA />
<section begin=AUS /> Australia's final squad was announced on 1 July 2021.
<section begin=AUSWPT />Head coach: Elvis FatoviÃÂ
<small>Note: Age as of 23 July 2021<br />Source: Australia Men | Tokyo 2020 Olympics </small><section end=AUSWPT /><section end=AUS />
<section begin=CRO /> Croatia's final squad was announced on 8 July 2021.
<section begin=CROWPT />Head coach: Ivica Tucak
<small>Note: Age as of 23 July 2021<br />Source: Croatia Men | Tokyo 2020 Olympics </small><section end=CROWPT /><section end=CRO />
<section begin=KAZ /> <section begin=KAZWPT />Head coach: Nemanja Kneà ¾eviÃÂ
<small>Note: Age as of 23 July 2021<br />Source: Kazakhstan Men | Tokyo 2020 Olympics </small><section end=KAZWPT /><section end=KAZ />
<section begin=MNE /> Montenegro's final squad was announced on 8 July 2021.
<section begin=MNEWPT />Head coach: Vladimir GojkoviÃÂ
<small>Note: Age as of 23 July 2021<br />Source: Montenegro Men | Tokyo 2020 Olympics </small><section end=MNEWPT /><section end=MNE />
<section begin=SRB /> Serbia's final squad was announced on 8 July 2021.
<section begin=SRBWPT />Head coach: Dejan SaviÃÂ
<small>Note: Age as of 23 July 2021<br />Source: Serbia Men | Tokyo 2020 Olympics </small><section end=SRBWPT /><section end=SRB />
<section begin=ESP /> Spain's final squad was announced on 9 July 2021.
<section begin=ESPWPT />Head coach: David MartÃÂn
<small>Note: Age as of 23 July 2021<br />Source: Spain Men | Tokyo 2020 Olympics </small><section end=ESPWPT /><section end=ESP />