Forbes netted his sixth goal of the campaign from the spot after Jim O'Brien had mesmerised Sean Dillon into bringing him down in the box with 24 minutes remaining.
On-loan Everton goalkeeper Ruddy then pulled off a simply unbelievable triple-save to deny Darren Dods, Danny Cadamarteri and Andy Webster, all from point-blank range, as United blew the chance to go level with Rangers at the summit.
It was Boys' Own stuff from the 20-year-old, and it will be difficult to find a better intervention from any keeper all season.
United were forced into two midfield changes today due to illness and suspension.
Morgaro Gomis failed to shake of a virus, and was replaced by Craig Conway, while David Robertson came in for banned namesake Scott.
Motherwell, who won 4-0 on their last visit to Tannadice, drafted in Steven Jennings for Jamie Murphy in the only alteration from last weekend's goalless draw with Rangers.
Neither side gave an inch in a competitive opening 10 minutes that was punctuated by a number of meaty tackles.
Referee David Somers had his whistle in his mouth plenty of times but chose to keep his cards in his pocket.
Defences were well on top and a goal only looked like coming on the break, with Cadamarteri's right-wing cross just too high for the unmarked Conway after a swift United counter.
But 24 minutes in, the home side almost scored a comical own goal when a mix-up between Dillon and goalkeeper Nicky Weaver almost saw the defender's header back end up in the net.
At the other end, David Goodwillie was put under too much pressure to be able to connect properly with Cadamarteri's latest cross.
It took half an hour for Somers to flash a yellow card, Jennings cautioned for an obstruction that left Conway in a heap.
Jennings was almost immediately substituted for Lukas Jutkiewicz, who won a 35th-minute free-kick which Weaver was forced to punch.
Three minutes later, the United keeper was finally called into meaningful action, matching Jutkiewicz's stunning 20-yard half-volley with a save high to his left.
United broke from the resulting corner but Goodwillie's finish was off target after a fine run.
A late tackle on O'Brien led to a booking for Prince Buaben, who was hurt himself moments later by a Jutkiewicz challenge which brought a free-kick but nothing more.
On-loan Everton forward Jutkiewicz was involved again shortly after the restart in an innocuous-looking incident that left Paul Dixon with a cut above his eye.
Left-back Dixon was replaced by Mihael Kovacevic in the 50th minute, with Dillon switching flanks.
Both teams' final passes continued to let them down and with less than half an hour remaining, United threw on summer signing Jennison Myrie-Williams for Danny Swanson on the right wing.
But United shot themselves in the foot in the 66th minute when Dillon tripped O'Brien in the area.
O'Brien missed a late penalty to beat Rangers last week, but this time regular taker Forbes stepped up and beat Weaver low to his left.
Dods, Webster and Cadamarteri all looked certain to level with 20 minutes remaining, but Ruddy had other ideas.
United made their last change by throwing on Andis Shala for Conway.
Motherwell defender Steven Hammell was booked for time-wasting in the closing stages but was then well within his rights to stay down after a horribly late tackle from Goodwillie, who was also carded.
Ruddy fittingly had the final say, palming over Buaben's 30-yard strike which was heading for the top corner.