New signing John Sutton had given the home side the lead with a debut goal 12 minutes from the interval but the sides were on level terms by the break after Tannadice captain Lee Wilkie headed home a close-range equaliser.
Motherwell dominated the second half but striker Porter failed to cash in from the spot after being brought down.
The draw gave Well their first point of the season, but both sides are still looking for their first win after three games.
Sutton's inclusion was one of two changes made by Motherwell manager Mark McGhee - Steven McGarry also came into the side - with Stephen Hughes missing out due to a hamstring injury and Jamie Murphy dropping to the bench.
There was one change to the United team which drew with Celtic last weekend, Prince Buaben starting in place of David Robertson, who was on the bench.
Buaben had the first goal effort of the match with a 25-yard diagonal shot from the left which home goalkeeper Graeme Smith stretched full-length to beat round the opposite post, the resultant corner being easily cleared.
United carried the greater attacking threat in the opening 15 minutes, with Morgaro Gomis looking particularly lively in linking the visiting midfield and forward line.
Motherwell's first real chance came when McGarry floated in a ball from the right flank on 17 minutes intended for Porter at the far post, only for Michael Kovacevic to muscle the home striker off the ball; and John Sutton was then flagged offside from a David Clarkson break.
Gomis came agonisingly close to giving United the lead on 23 minutes, but he struck the right-hand post when he had looked sure to score with a shot from Warren Feeney's pass.
Motherwell earned a free-kick just before the half-hour and McGarry swung in the 25-yard effort to the opposite post, but goalkeeper Zaluska was equal to Sutton's header.
The home side enjoyed more possession and forward play as the half wore on, and took the lead after 33 minutes when Sutton marked his arrival with a debut goal.
Clarkson began the scoring move by switching play to Paul Quinn on the right.
Quinn cut in to the edge of the box and crossed at the second attempt to Keith Lasley.
The midfielder headed the ball back into the middle, where Sutton was waiting to bundle home from close range.
However, the lead lasted just six minutes before United got back on level terms.
Gomis fired in an inviting cross from the left corner and centre-half Wilkie was on hand to head home from his station beside the opposite post.
Motherwell started the second half in positive style and were inches away from regaining the lead following a Steven Hammell free-kick in midfield after Buaben had brought down Porter.
The set piece was glanced on by Quinn and then Sutton almost on the goalline, but was turned just past for the hosts' third corner in the five minutes since the restart.
The home side laid constant siege to the United goal and were incredulous after failing to score in near-comical fashion in the 57th minute.
Hammell lofted a long ball half the length of the field and Clarkson was left in plenty of space to bear down on goal.
Zaluska stretched up a hand to stop his initial lob, and Clarkson then contrived to hit the bar from close range with his second attempt, and neither he nor strike partner Porter were able to turn the ball home as it bobbled around the six-yard box.
United goalkeeper Zaluska then maintained his excellent second-half form by decisively saving a penalty from Porter on the hour.
The Motherwell striker had been crudely bundled over by Garry Kenneth as he raced into the area and stepped up to take the spot kick himself.
But Zaluska was more than equal to the task as he got down to his left to beat the ball to safety.
Quinn played a good one-two with Marc Fitzpatrick on the edge of the box and Porter and Sutton then did the same inside the six-yard area, with Porter finally turning his close-range effort wide of the near post.
Francesco Sandaza provided some menacing moments for the home defence late on, especially when he linked with Gomis and Buaben on the edge of the box.
Motherwell continued their relentless search for an equaliser in the closing stages, but Porter was flagged offside when he thought he had netted the winner from Clarkson's through ball nine minutes from time.
Fitzpatrick blasted off target in injury-time, while Buaben saw a last-ditch diagonal effort diverted out for a corner.