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). He was voted in the All-Tournament Team of the 2015 World Championship as well as the All-Tournament Team of the 2017 World Championship. Fountoulis has been the (2012–2017, consecutive) playing for Olympiacos. He has been voted MVP of the Greek Water Polo League τhree times in 2012–13, 2014–15 and 2016–17 seasons and three times Greek Cup MVP in 2011, 2013 and 2018.