Roethlisberger missed one series because of a left knee injury and was held to 189 yards passing, and Miami won 30-15 Sunday.
Jay Ajayi rushed for 204 yards and two scores on 25 carries for the Dolphins (2-4).
''No need to sugarcoat,'' Pittsburgh coach Mike Tomlin said. ''We got beat soundly.''
The Steelers (4-2) had scored 74 points in their previous two games, but were outgained 474-297. Miami quarterback Ryan Tannehill said he was buoyed to see two Pittsburgh defenders become sick to their stomach late in the first half as players dealt with the 80-degree heat.
''It's definitely a glimpse of what we can do,'' Tannehill said. ''Now we just have to show up every week like this.''
Roethlisberger finished 19 for 34 with two interceptions. He hobbled to the locker room and missed one series late in the second quarter when he hyperextended his knee scrambling on a play that resulted in an interception by Reshad Jones.
After the game, Roethlisberger had ice on both knees and a bruise in the middle of his back. An MRI on his knee was planned.
''It happened before I threw the ball,'' Roethlisberger said. ''I felt something funny in my knee. We'll pray it's nothing too serious.''
Roethlisberger also tweaked his right knee on the Steelers' final play.
He had only 83 yards passing with 5 minutes left before he drove Pittsburgh 97 yards for a touchdown that made it 23-15. Miami recovered the ensuing onside kick, and on the next play Ajayi scored on a 62-yard run.
Pittsburgh's only other touchdown came in the first quarter when Darrius Heyward-Bey scored on a 60-yard end around. Antonio Brown, the NFL's leading receiver, was held to four receptions for 39 yards. Teammate Sammie Coates, playing with a hand injury, didn't have a catch.
The Dolphins were one-touchdown underdogs at home, but the score could have been more lopsided had they not repeatedly squandered chances in the red zone. They had a 13-minute advantage in time of possession in the biggest win yet for first-year coach Adam Gase.
''I think what today proved to a lot of our guys and coaches what you possibly could be,'' Gase said. ''But this league is week to week. Once we hit next Sunday, nobody is going to care about what we did this week.''
The Dolphins started the game with their top five offensive linemen on the field together for the first time this year. How much difference did that make?
''You tell me,'' Gase said. ''A lot, right? It makes a big difference when we have holes and the quarterback is not on his back.''
The Steelers failed to sack Tannehill after he was taken down six times a week earlier against Tennessee. Second-year pro Ajayi took advantage by surpassing 100 yards rushing for the first time.
Tannehill, booed by fans a week ago, drew only cheers while going 24 for 32 for 252 yards, including a pair of long throws.
He slipped from the grasp of 352-pound Daniel McCullers to throw a 53-yard pass to MarQueis Gray that set up a field goal. Tannehill's 39-yard pass to Jarvis Landry set up Miami's first touchdown with 21 seconds left in the first half for a 16-8 lead.
The crowd appeared evenly divided early, with many fans waving yellow towels for every play that went the visitors' way. But as Miami pulled away in the second half, it seemed the entire stadium was singing the Dolphins fight song.
Twice Miami settled for a field goal after it appeared Tannehill had thrown a touchdown pass. DeVante Parker dropped a throw in the end zone, and a TD catch by Dominque Jones was negated by a penalty for illegal formation.
Andrew Franks kicked three field goals, but missed a 50-yarder and had a 24-yarder blocked.
Dolphins: CB Tony Lippett left with a hamstring injury. TE Dion Sims was evaluated for a possible concussion.
AP NFL website: www.pro32.ap.org and https://twitter.com/AP-NFL
Follow Steven Wine on Twitter: https://twitter.com/Steve-Wine. His work can be found at http://bigstory.ap.org/content/steven-wine