Italy striker Belotti broke a league scoring duck which stretched back to May last year when he opened the scoring in the 17th minute at a packed Stadio Olimpico.
And after Antonio Candreva put the away side in the lead with two beautiful strikes either side of half-time, Belotti then powerfully headed Roma level from Leandro Paredes' corner with eight minutes remaining.
Roma are looking for a new striker as Tammy Abraham will be out until January, but Belotti wasn't disheartened by having a seventh-minute finish disallowed for his toe straying offside as Bryan Cristante clipped over a perfect long pass.
Ten minutes later the 29-year-old was finally celebrating a league goal after holding off former Roma man Federico Fazio and two more Salernitana defenders before poking past Guillermo Ochoa.
Mexican goalkeeper Ochoa stopped Roma from doubling their lead in the 33rd minute when he brilliantly kept out a Chris Smalling header and Gianluca Mancini couldn't force home from close range.
Candreva took advantage to level the scores moments later, smashing home after leaving Smalling on the ground with a nifty feint.
And the 36-year-old left fans in the Olimpico open-mouthed shortly after the break by putting the away side ahead with an even better goal.
Collecting a Domagoj Bradaric crossfield pass on the corner of the penalty area, Candreva cut inside to beat Cristante before unleashing a dipping strike with his left foot which left Rui Patricio grasping at thin air.
Roma pushed for an equaliser and got a deserved point thanks to Belotti, who looked ready to battle whoever the club bring in as competition before the end of this month.
De Ketelaere didn't score a single goal for AC Milan last season after signing for 35 million euros from Club Brugge last summer.
But the Belgium international got off to the perfect start as an Atalanta loanee, heading in the opening goal in the 83rd minute before Nadir Zortea lashed in the away team's second in stoppage time.
De Ketelaere also struck the woodwork after coming on as a second-half substitute for Gian Piero Gasperini's side, who were dominant at the Mapei Stadium.
Sassuolo were without star man Domenico Berardi whose hoped move to Juve, who are at Udinese later on Sunday, fell through in the week.
CEO Giovanni Carnevali said before kick-off that Berardi would not be leaving for Juve as Sassuolo wouldn't have time between now and the end of the transfer window to find a replacement.
Last season's runners-up Lazio begin their campaign at Lecce in the other of two late matches.
