Hamilton was coming through the field on an alternate tyre strategy, attacking McLaren's Piastri for eighth on his fresher mediums on lap 41.
Hamilton passed Piastri on the inside of the second chicane, but squeezed the Australian towards the grass, his right rear tyre making contact with Piastri's front wing.
After both drivers took to the escape road Piastri had to pit for a new front wing, losing a points finish.
Hamilton received a five-second penalty for causing an avoidable collision, which didn't end up costing him a position as he remained sixth.
Afterwards the seven-time world champion owned up to his mistake and apologised to Piastri for the incident.
"I apologised because it was obviously my fault," he said. "And it naturally wasn't intentional. I got up alongside and just misjudged the gap that I had to the right, clipped him.
"It could happen anytime. But I knew shortly afterwards, it must have been my fault.
"I wanted to make sure he knew that it was intentional. And that's what gentlemen do, right?"
Piastri, who also made contact with team-mate Lando Norris and AlphaTauri's Liam Lawson in an action-packed afternoon, accepted Hamilton's apology and was keen to move on.
"I think [he] just moved a bit too far to the right, but I think it's very easy to do in that corner. It's very narrow," Piastri said after dropping down to 12th.
"He came and apologised, so I don't think there's much more to it than that."
McLaren team principal Andrea Stella similarly appreciated the apology from Hamilton, who he felt was a bit too impatient as he would have come through anyway.
“I think maybe Lewis got a little frustrated because in reality, if not there Lewis would have overtaken Oscar at the next opportunity," Stella added.
"So, there was no need to take too much risk for Lewis because he had the pace to overtake with more margin.
"Lewis is a fair competitor, if he apologised, it shows his qualities as a competitor and as a person. He definitely has our respect.
"We just ended up out of the points when we could have had two more points today and this is a shame because our objective is to score points with two cars."
