In a nationally televised verdict, the Seoul Central District Court convicted Park of bribery, extortion, abuse of power and other charges.
"The defendant abused her presidential power entrusted by the people, and as a result, brought massive chaos to the order of state affairs and led to the impeachment of the president, which was unprecedented," judge Kim Se-yoon said as he handed down the sentence.
The court ruled that Park colluded with her old friend, Choi Soon-sil, to receive millions of dollars from major conglomerates such as Samsung and Lotte to help Choi's family and fund non-profit foundations owned by her.
The court said Park also colluded with senior government officials to blacklist artists critical of her government and passed on presidential documents with sensitive information to Choi via one of her presidential aides.
The scandal has already led to the arrests, indictments and convictions of dozens of high-profile government officials and business leaders.
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The judge said Park had shown "no sign of repentance" but had instead tried to shift the blame to Choi and her secretaries.
"We cannot help but sternly hold her accountable," Kim said.
Along with the prison sentence, Park was also fined $16.8 million, Kim said.
Park was removed from office early last year following months of massive rallies that saw millions take to the nation's streets calling for her ouster. That led to a presidential election won by the liberal Moon Jae-in, whose conciliatory stand on North Korea has underpinned a significant warming of ties between the rival neighbors.
Moon's office said Park's fate was "heartbreaking" not only for herself but for the country, and added that history that was not remembered would be repeated.
"We will not forget today," the office said.
Up to 1,000 Park supporters gathered outside the court, holding national flags and signs calling for an end to "political revenge" against her.
Park has been held at a detention center near Seoul since her arrest in March 2017, but she refused to attend Friday's court session, citing sickness.
The conviction, which she can appeal, is the latest hit in a dramatic fall for South Korea's first female president. Once seen as the darling of South Korean conservatives, she was dubbed "Queen of Elections" by local media for her track record of leading her party to victory in tight races and still has a small group of fierce supporters who regularly stage rallies calling for her release.
Park, 66, maintains that she's a victim and has been refusing to attend court sessions since October.
A jail sentence will be a bitter blow for the daughter of a former military dictator, who returned to the presidential mansion in 2012 as the country's first woman leader, more than three decades after she left it following the assassination of her father.
Park's friendships with Choi, 61, began in the mid-1970s when Choi's late father served as Park's mentor after her mother's assassination. Park once described Choi as someone who helped her when she had difficulties.
But her relations with the Choi family have long haunted her political career. Media reports say that Choi's father was a cult leader and allegedly used his ties with Park to take bribes from government officials and businessmen.
Park has legions of loyal supporters, most of them older conservatives who remember her father's authoritarian 18-year rule, beginning in 1961, when their country began its remarkable surge toward becoming an economic power.