The game should have strengthened United's case for holding onto third place this season but they were instead left reeling as Marc Fitzpatrick, Maros Klimpl, David Clarkson and Chris Porter all helped themselves to a goal.
Motherwell plunged as low as joint-bottom of the SPL earlier this season but three wins from four games means they have now hauled themselves into mid-table and could now even be belated challengers for the 'best of the rest' tag they claimed last term.
It was an entertaining encounter right from the outset. Steven Hammell earned himself a yellow card less than a minute after kick-off for hauling down Warren Feeney on the edge of the box, allowing United to pile on the pressure early on.
Paul Dixon stepped up for the free-kick and struck the ball well enough but watched in frustration as his effort flashed across the face of goal and wide of target.
At the other end, Porter claimed his jersey was tugged by Lee Wilkie but his pleas were ignored by referee Eddie Smith.
Motherwell goalkeeper Graeme Smith was then called into action to block a right-footed volley from Feeney in front of goal after he had been set up by Morgaro Gomis.
Feeney was threatening again moments later, this time throwing himself to meet a Craig Conway cross and glancing his header just wide of the upright.
Despite boasting the best of the chances earlier in the game, United found themselves trailing with 23 minutes gone.
Paul Quinn delivered an excellent cross from the right flank across the face of goal where Fitzpatrick was waiting in plenty of space to nod past goalkeeper Lukasz Zaluska.
The United keeper then pulled off a superb save to deny a left-footed volley from Klimpl but was left plucking the ball out of the back of the net again just four minutes after the opener.
Klimpl's thunderous effort from the edge of the box was cleared but he pounced on the rebound to send a firm shot into the bottom corner of the net.
United were clearly rattled and Motherwell raced into a three-goal lead with 33 minutes on the clock. This time, Jim O'Brien slipped the ball through to Clarkson and he rifled clinically into the back of the net.
Tangerines boss Craig Levein attempted to whip up support from the Tannadice crowd as his players left the field at half-time but it was Motherwell who almost found the back of the net again early in the second half.
Clarkson turned provider when he made a run to the by-line before squeezing the ball across goal to Porter for what would have been a simple tap-in but he failed to connect at full-stretch and the danger was cleared.
The game was growing more heated and Garry Kenneth picked up the second booking of the match for a crunching tackle on Porter with just over an hour gone.
Any hopes of a United comeback were dashed after 63 minutes. Substitute Danny Swanson lost possession to Fitzpatrick who teed up the shot for Porter to lash into the bottom corner from 12 yards.
Francisco Sandaza was thrown into the action to bolster the home side's hopes of a belated fightback. But United's attempts to push forward and pull a goal back were easily snuffed out by a Motherwell side whose confidence was now sky-high.
The visiting fans shouted for a fifth goal but were no doubt more than happy to return to Fir Park with a surprisingly comfortable victory.