Hard to find records, but the earliest non-US NBA player was Hank Biasetti, who was born in Italy and grew up in Canada. He played at a Canadian College (Assumption) in 1942.

Gino Sovran was another Canadian who started at Assumption, but then transferred to Detroit Mercy (an NCAA school) in 1945. That was 16 years before Miko Simon (Finland) took the court for BYU, reported as the first overseas player.

Prior to that there were also a number of other Canadian players, as well as German or Dutch nationals who grew up in the United States, both of which would be considered international, but not necessarily overseas players