A minute earlier, the two attacking stars linked up to put Juve level at 2-2.
Maurizio Sarri's men were set to pay a hefty price for a poor performance until Dybala appeared off the bench and won a 68th-minute spot-kick, which Ronaldo duly converted.
Leonardo Bonucci had fired Juve into a 20th-minute lead but an almost immediate response from Jeremie Boga seem to send panicked shivers through the home side.
Francesco Caputo capitalised on dual defensive blunders to give Sassuolo the lead and, though Juve steadied enough to pinch a point, the result would have been appreciated by Inter, who had SPAL at home later on Sunday.
Coach Sarri heeded Emre Can's public plea for a start and the midfielder's first major act was to miscue a straightforward finish.
Gonzalo Higuain stabbed a tougher chance past the post before captain Bonucci set the example.
The centre-back's ambitious drive whistled past teenage debutant Stefano Turati in the Sassuolo goal and gave Juve a lead that lasted barely two minutes.
The equaliser arrived in style as Boga produced a divine dink over the onrushing Gianluigi Buffon after Caputo's pass had nutmegged Alex Sandro.
Italy great Buffon was beaten again within two minutes of the restart, this time from an entirely less glamorous finish.
Matthijs de Ligt's rash clearance was sent back into the box and Caputo's tame attempt squirmed through a red-faced Buffon.
Turati proved more assured as he acrobatically parried Ronaldo's free-kick, before Higuain and Can squandered gilt-edged opportunities.
Ronaldo had a goal correctly disallowed for offside but was not to be denied from the spot, the 34-year-old confidently converting after Filippo Romagna brought down Dybala.
Juve looked likely to go on and snatch a winner, but a Dybala shot that drew an excellent save from Turati and a Ronaldo header that looped wide proved the closest they came.