A long ball over the top from Davinson Sanchez should have been easily dealt with by Red Devils full-back Diogo Dalot.
But Zaha, who signed for Galatasaray in the summer after leaving Crystal Palace, wrestled with the United defender and got the better of him.
A speculative shot hit Dalot's thigh and bounced over Andre Onana, who was off his line and could not reach the ball.
It saw Galatasaray grab an equaliser after Rasmus Hojlund had earlier put United ahead at Old Trafford.
And the Ivory Coast international made sure the home crowd were reminded of who he was after his short stint at the club.
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Taunting the fans, he cupped his ears to them and roared in celebration.
Zaha made just appearances for United after signing for them in 2013, being loaned out to Palace and Cardiff before Louis van Gaal sold him back to Selhurst Park.
And after failing to score in 11 appearances against his old side, he has since netted four in his last five.
This is the first time that Zaha has started for Galatasaray in the Champions League, grabbing his first goal in the competition as a result.
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The winger left Palace in the summer after nine years at the club in pursuit of Champions League football.
He signed for Galatasaray, who then drew United in the group stage - and this match was surely marked on his calendar.
The Turkish side have made some big moves in the transfer window, taking several Premier League exports.
As well as Zaha from Palace and Sanchez from Tottenham, Hakim Ziyech signed for the club on loan from Chelsea.
Tanguy Ndombele also joined on loan from Spurs while Angelino, Tete and Lucas Torreira all have experience of the Premier League.