Trailing 25-19 with 2:18 left in the game, the Steelers took over at their own 20-yard line. After completing a 19-yard pass to Antonio Brown, Ben Roethlisberger's next attempt was picked off by Porter, who scampered into the end zone untouched.
Peyton Manning executed 19-of-26 passing efficiency for 253 yards and two touchdown passes in his debut for Denver (1-0) after signing with the Broncos as a free agent in March.
His 1-yard toss to Jacob Tamme and the following 2-point conversion with 9:23 left put Denver ahead for good at 22-19.
Demaryius Thomas made five grabs for 110 yards and a score, while Eric Decker also hauled down five passes for 54 yards for the Broncos.
Roethlisberger completed 22-of-40 attempts for 243 yards with two TDs in addition to the pick for the Steelers (0-1).