Kostas, along with hockey player Filip Forsberg and three of his brothers, Giannis, Thanasis and Alex, were announced as joining the Nashville SC ownership group in the MLS.

[5] He has a Nigerian passport, granted to him in June 2013 so that he could gain a visa, and be allowed to legally enter the United States.

In the 2016–17 season, his first with the Flyers, he was red-shirted,[11] after being ruled a "partial qualifier",[12] due to having spent his first two years of high school in Greece.

Throughout the contract, he would split his playing time between the Mavericks and their NBA G League affiliate, the Texas Legends.

[20] During his time in the G-League, he averaged 12.3 points and 7 rebounds per game, earning him the nickname “Mr Fantastic” by Legends Radio Announcer Quinn Redmond.

[23][24] He played 5 games throughout the season on a two-way contract and scored a career high 7 points in the team's 136–122 loss to the Sacramento Kings on August 13, 2020.

[29] On December 19, 2022, Antetokounmpo signed with Fenerbahçe Beko of the Turkish Basketball Super League (BSL).

On June 22, 2023, Antetokounmpo signed a two-year contract with Panathinaikos of the Greek Basket League and the EuroLeague, where his older brother Thanasis had previously played for two seasons.

[31] On February 5, 2025, coach Ergin Ataman announced that Antetokounmpo would be parting ways with the defending EuroLeague champions.

[34] Antetokounmpo played with the senior men's Greek national team at the 2020 Victoria FIBA World Olympic Qualifying Tournament, and at the EuroBasket 2022.