Also on Tuesday, billionaire businessman Shin Dong-bin, chairman of conglomerate Lotte, was sentenced to 2.5 years in jail on related bribery charges. Former presidential secretary An Chong-bum got a six-year term and was fined US$92,000.
Lotte's chairman, Shin Dong-bin, was in court at the time of the verdict and was taken into custody.
A confidante of former South Korean President Park Guen-hye has arrived in court to hear the verdict in her political corruption case.
Choi tearfully apologized when she was summoned by prosecutors in October 2016 - her first public appearance - as the fury over the scandal sparked nationwide protests urging Park's ousting.
Choi also was found guilty of using her relationship with Park to force large businesses to donate more than $68 million to two non-profit corporations she controlled.
Ms Park was officially ousted in March 2017, following parliament's decision to impeach her. Headed by Choi, who leveraged her Blue House connections, these extracted approximately US$70 million from big business.
Yet, since Korea's first Olympic hosting in 1988, conglomerates have played a significant role in sports and cultural promotions, usually hand-in-hand with government.
Choi Soon-sil, center, a confidante of former South Korean President Park Geun-hye, arrives at the Seoul Central District Court in Seoul, South Korea, Tuesday, Feb. 13, 2018. Suspicions snowballed into allegations that Choi was central to national governance.
Former South Korean president Park Geun-hye arrives at court on August 25, 2017. The de-facto head of technology giant Samsung Electronics was jailed past year in a related trial but he was unexpectedly released last week on appeal, in a ruling that was perceived as a setback to government pledges to curtail the power of the nation's corporate elite.More news: Unilever's warning to tech giants: Drain the swamp or lose advertising
"The accused is sentenced to 20 years in prison", judge Kim Se Yoon said.
Choi's lawyer vowed to appeal. "This is a heavy sentence bordering on cruelty", he told local reporters.
She feared that if Park became head of state, "the country would be ruled by the collective leadership of the Choi clan", the former journalist wrote.
It said her crime was grave given how she triggered an unprecedented impeachment of the president and disappointed the public.
That responsible party is clearly Choi - who extracted the bribes - rather than the parties who offered them.
Shin was given a light sentence and Samsung's Lee has successfully appealed his five-year jail term in the case. Shin was sentenced to two years in prison.
Meanwhile, Park herself has been in detention since being booted from office and is on trial. She was introduced by her father to Park years earlier.
The senior Choi, 40 years older than Park and the seven-times-married founder of a cult-like group, won her trust.