The Biebs was spotted out Monday on a jet surfboard zipping around Puslinch Lake in Ontario, Canada, where he's hanging out during this quarantine. Witnesses tell us he surfed the perimeter of the lake and waved hello to neighbors.

We're told the wifey, Hailey, was also hanging out on the dock watching Mr. Bieber enjoy the hell out of his quarantine time while staying at a house near the lake, in isolation, during this pandemic.

In any event ... at least these pics show Justin's like the rest of us trying to enjoy some time outdoors after being holed up inside for weeks now. He's taking part in the famous All In Challenge ... with a chance for him to give one lucky fan a one-on-one concert.

Surf's up, dude.