Dundee secured their first derby win at Tannadice since 2004 in a pulsating six-goal thriller.
The Dark Blues went in at the break 3-1 up thanks to a Jordan McGhee double and another goal by Scott Tiffoney, with Glenn Middleton on target for the hosts.
United scored again through Kristijan Trapanovski at the start of the second half but Simon Murray sealed the win for the visitors from the penalty spot in stoppage time with Terrors substitute Emmanuel Adegboyega shown a straight red card.
The vital victory takes Dundee five points clear of bottom club St Johnstone, while United’s top-six and European hopes have suffered a blow.
The Dark Blues’ support were celebrating in the 11th minute when Lyall Cameron fired home but after a VAR check his effort was ruled out as Seun Adewumi was offside in the build-up.
However, there was no VAR intervention when Dundee took the lead at the second time of asking in the 17th minute, with Ziyad Larkeche hitting a superb cross from the left and McGhee stooping to head past home goalkeeper Jack Walton.
Dundee’s tails were up and they doubled their advantage in the 27th minute. Larkeche found Tiffoney who ran to the edge of the United box and hit a shot which squirmed through Walton’s grasp and into the back of the net.
However, United hit back in the 31st minute. Aaron Donnelly conceded possession to Craig Sibbald and he passed to Middleton with the winger hitting an unstoppable shot into the top corner.
Murray was then targeted by a vape thrown from the home support with the furious striker having to be restrained by his team-mates.
Dundee restored their two-goal advantage in the 39th minute and it was another cracker. Adewumi cut the ball back to McGhee on the edge of the box and the defender sent a superb curling shot into the back of the net.
The Dark Blues thought they had scored a fourth shortly afterwards thanks to a Ross Graham own goal but for the second time in the afternoon it was ruled out by VAR for an earlier offside by Murray.
The home fans would have been looking for a big response from their team after the break and they did not have to wait long with United making it 3-2 in the 49th minute. Sam Dalby found Trapanovski inside the Dundee box and he hit a shot back across goal and past Trevor Carson.
Nerves were jangling in the closing stages with home substitute Louis Moult having a great chance but his header was straight at Carson.
Instead, it was Dundee who sealed the win in the seventh minute of stoppage time.
Murray was bundled over in the penalty area by Adegboyega as he raced through on goal, with the defender shown a red card by referee Nick Walsh. Murray picked himself up and calmly dispatched the spot-kick.
