Liberals said the embarrassment suffered by Nazril "Ariel" Irham - who insists the videos were not intended for public viewing - was punishment enough. But hard-liners were outraged, saying the singer was contributing to the country's moral decline.
Hundreds charged the gates of the courthouse in the city of Bandung after the verdict was read out, yelling "Too light! Too light," as he sped off in an armored police car.
Police fired warning shots to break up a scuffle between his supporters and critics.
Ariel, lead singer of the country's most popular band, Peterpan, was the first celebrity to be charged under Indonesia's strict anti-pornography law, which came into effect in 2008 despite strong opposition from the public and members of government.
It is seen by many as vaguely worded and as carrying overly harsh penalties.
Ariel insists the tapes were stolen from his house and posted online without his knowledge, but presiding Judge Singgih Budi Prakoso said the pop star did nothing to prevent them from spreading online.
He sentenced him to 3½ years in jail - well short of the maximum 12 years - and slapped him with a $25,000 fine.
"As a public figure, the defendant should be aware that fans might imitate his behavior," Prakoso said, adding that the frontman for Peterpan - which is also popular in neighboring Malaysia, Singapore and Brunei - did not show remorse.
His supporters were devastated.
"He's just a victim," said Roslawati, 30, as tears streamed from his eyes. "He didn't post that video."
Indonesia, a secular country of 237 million people, has more Muslims than any other country in the world. Though most are moderate, a small extremist fringe has become more vocal in recent years.
They have pushed through controversial laws - including the anti-porn bill - and been known to attack anything perceived as blasphemous, from transvestites and bars to "deviant" religious sects.
More than 500 demonstrators turned out Monday, some pelting the police vehicle carrying Ariel to the court with rotten eggs and tomatoes. Others held placards criticizing the star.
When the verdict was read out, they rushed and rattled the gates of the courthouse.
"More years for Ariel!" shouted Kurnia Maryati, a 33-year-old mother of three who was wearing an Islamic headscarf.
The videos were made public in June.
The first six-minute clip purportedly showed Ariel in bed with his girlfriend, Luna Maya, a top model and actress who joined him at the court Monday. She broke down in tears when the ruling was read out.
The second clip showed him with a former girlfriend, also a well-liked model and television presenter, Cut Tari.
The clips were spread on Facebook and YouTube and distributed by mobile phone rapidly, prompting the government to try to impose a so-far unsuccessful online filter for porn.
It is not yet clear if the women in the videos will also be punished.
But the man who admitted to posting the tapes on the Internet was sentenced Monday in a separate trial to two years in jail.