Zac Efron Posts For First Time Since Hospitalization, Says He's 'Healthy'

Zac Efron says he's feeling fine after an overnight stay in the hospital ... posting a workout pic to show fans he's already back up and lifting.

The actor shared the snap to his Instagram story just minutes ago ... chest-pressing some pretty major dumbbells while lying atop a yoga ball -- implying he's not taking any days off from the grind.

Efron writes he's "happy and healthy" and thanks everyone for their well-wishes. He even adds the date -- doing everything to let fans know they took the photo earlier Sunday.

Of course, our sources tell us Zac went through a pretty scary situation Friday ... with a couple employees at a private villa in Ibiza having to pull him out of a pool after seeing him in distress.

Despite sounding pretty serious, a rep for Zac told us he was only taken to the hospital as a precaution after a "minor swimming incident" ... though the rep wouldn't explain what caused him to be in distress in the pool.

Like we told you ... Zac's been vacationing in Europe -- recently hitting the Paris Olympics and linking up with Martin Garrix for a DJ set -- and, he's putting fans' minds at ease with two dumbbells up.