A Seoul Central District Court judge delivering its ruling on bribery and other charges after a six-month trial in a scandal that triggered Park's impeachment and ouster, also found Lee guilty of hiding assets overseas and perjury.
Mr. Lee, who also goes by the name Jay Y. Lee in the West, had survived the prosecutor's first attempt to arrest him last month when a court in Seoul ruled that there was not enough evidence of bribery.
The Seoul Central District Court said Samsung's financial support of entities backed by Park's close friend, Choi Soon-sil, constituted bribery, including 7.2 billion won ($6.4 million) in sponsoring the equestrian career of Choi's daughter.
Former Samsung Electronics President Park Sang-jin received a three-year jail term, suspended for five years, and former Executive Vice President Hwang Sung-soo was sentenced to 2 1/2 years, suspended for four years.
Song Wu-cheol, Lee's lawyer, said that the defence intends to appeal the decision, telling Reuters that the verdict is "unacceptable" and that the defence is confident the ruling will be overturned.
"As a lawyer I can not possibly agree with the juridical decisions and acknowledged facts involved in the verdict", Samsung attorney Song Woo-cheo said outside the court.
Lee Jae-Yong, the son of Samsung group chairman Lee Kun-Hee, has been groomed all his life to take over the giant conglomerate founded by his grandfather in 1938. Korea's National Pension Service, a multi billion-dollar fund with money from millions of citizens, cast a key vote in favor of that 2015 merger.
"I think the court should have accepted prosecutors' request for 12 years in prison", said Choi Sukun, a 37-year-old teacher in Seoul.
Mr Lee also joins the list of Korean business leaders to be convicted of corrupt behaviour, one that already includes his father Lee Kun-hee. Park allegedly gave Choi access to government documents for which she had no clearance. But even if convicted, many see their sentences significantly reduced on appeal or suspended, leaving only a few actually spending significant time behind bars.
Samsung says the Lee family isn't involved in day-to-day operations, leaving those responsibilities to trusted lieutenants.
According to the news agency hundreds of diehard supporters of the former president reacted with outrage on Friday after the court's verdict. "This seems to be the court system simply following the constitution and administering the law".
Lee has been Samsung's de facto leader since his father fell ill and the sentence leaves the conglomerate facing an extended vacuum at its highest level, but analysts differ on its impact.
Samsung Electronics shares closed 1.05 percent down on Friday after the verdict.