Mohamed Fares gave SPAL a 22nd-minute lead with a towering header from Thiago Cionek's cross.
Ranieri, put in charge until the end of the season after the departure of Eusebio Di Francesco, shook things up at halftime, bringing on Nicolo Zaniolo and Diego Perotti for Stephan El Shaarawy and Justin Kluivert.
The change paid off as Perotti converted a spot-kick in the 53rd minute after Edin Dzeko was brought down in the box.
SPAL, battling to stay clear of the drop zone, were not finished though and when Petanga was brought down in the box by Juan Jesus six minutes later he got up to fire home his penalty.
Dzeko was denied an equaliser by a goalline block but SPAL hung on for a precious three points.
Ranieri has been and began his tenure with a 2-1 win over Empoli. Roma stayed in fifth place, three points behind Inter who are in action in the Milan derby on Sunday.
Sampdoria beat Sassuolo 5-3 away in the earlier kickoff.
(Reporting by Martyn Herman, editing by Ed Osmond)
