The Norwegian nodded in a David Pizarro cross in stoppage time after a Francesco Totti penalty had cancelled out Alessandro Del Piero's fine opener for hosts Juve in the 51st minute.
The Turin side were down to 10 men when Roma scored the winner after goalkeeper Gianluigi Buffon was sent off in the 83rd minute for charging out of his area and fouling Riise.
The win saw Roma tighten their grip on third place with 38 points from 21 games, eight behind leaders Inter Milan, who face second-placed AC Milan in Sunday's high stakes derby.
"The victory was important because Juventus want to stay in the race for Champions League qualification and us winning puts them several points behind us. I'm not looking for revenge," Ranieri told Sky television.
The hosts had shown signs of recovering from recent woes, with Mohamed Sissoko's return from duty with Mali at the African Nations Cup strengthening their midfield.
"It's a difficult period, there's no use hiding it," Ferrara said. "Sometimes our defeats have been deserved, others not, but at this point we are a long way behind.
"I think the squad has the strength, character and personality to come out of this all together. In moments like these you have to be strong men and they are."
A section of Turin's Stadio Olimpico was closed during the match because of racist chanting by Juventus's ultra fans in an Italian Cup tie last week.
