Harlequins number eight Alex Dombrandt made a crucial interception on his own try line and ran 60 meters down field to set up a try at the other end of the pitch.
Ulster looked the more likely scorer at the Kingspan Stadium when the young back rower snatched possession for the visitors to set up a brilliant counter attack try for Harlequins.
The ball was eventually brought to the ground on Ulster’s 22-metre line when Quins hooker Elia Elia made a break down the blindside of the ruck to score his second try in three minutes.
The game finished in dramatic fashion when Ulster scrum half John Cooney kicked a late penalty in the 79th minute to win the game for the hosts.
The game finished 25-24 to Ulster leaving the Irish side top of Pool 3 in the Champions Cup after three rounds.
Full match highlights are available below: