Timothée Chalamet took a razor to his mane ... because he's showing off a new look ... and it's a shaved head!!!

The actor debuted his new hairstyle Tuesday on Instagram Live ... during a promotional spot for his new ping pong movie, "Marty Supreme."

In the footage, Timothée is walking around with his shaved head ... while wearing some "Marty Supreme" merch.

We saw Timothée and his girlfriend, Kylie Jenner , Monday night at the New York Film Festival ... and he was wearing a baseball cap ... and it looked like he might have buzzed off his hair.

Now we know for sure, and we gotta say ... it looks pretty good on him.

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