A high school basketball player wound up and sucker-punched his opponent twice during postgame handshakes this week ... and now cops are investigating.

Here's what we know -- Nevada H.S. was facing off against Carlisle H.S. in Iowa on Tuesday ... and when both teams lined up to say "good game" after Nevada's 72-47 win, #4 on Carlisle attacked #5 on Nevada.

Unclear what happened in the game, but it appears the players were going through the line without incident before Nevada's senior guard threw punches to the opponent's stomach and head.

VIDEO: https://www.tmz.com/2021/12/01/high-...me-handshakes/