[3][4] He played for the Greek powerhouse Olympiacos since 2009, and in 2018 he won the 2017–18 LEN Champions League with the Greek giants, scoring 4 goals in the final game against Pro Recco in Genoa.

Fountoulis was the top scorer of the 2017 World Championship with 23 goals and the top scorer of the 2018 European Championship with 17 goals (along with Aleksandar Ivović).

[12] Fountoulis has been the top scorer of Greek Water Polo League six times (2012–2017, consecutive) playing for Olympiacos.

[18][19][20] As of June 2018, Fountoulis has scored 148 goals in LEN Champions League, the most by any Greek player in the history of the top-tier European competition.

[21] Fountoulis was given the honour to carry the national flag of Greece at the closing ceremony of the 2020 Summer Olympics in Tokyo,[22] becoming the 28th water polo player to be a flag bearer at the opening and closing ceremonies of the Olympics.