Lee Jae-yong, heir apparent and de facto chief of global electronics giant Samsung, has been jailed for five years.
A South Korean court found him guilty of corruption in a case linked to disgraced former president Park Geun-hye.
Lee was accused of making US$36 million ($49.75 million) of illegal payments to a charitable foundation controlled by Park's close friend Choi Soon-sil.
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In return, he was alleged to have received political favours that included the ability to restructure Samsung's obscure shareholding structure and help the Lee family maintain control.
Lee denied all charges, and his lawyer said that he will appeal against the ruling.
The court ruling is seen as a high-profile slap to the chaebols, or multi-disciplinary conglomerates, that dominate Korean business and have long maintained cosy relationships with government.
The Lee family is also related to the founders of CJ Group, the foods-to-entertainment conglomerate that controls Korea's largest film and TV businesses CJ Entertainment and CJ E&M, as well as dominant cinema operator CJ-CGV.
As president, Park instituted a blacklist of entertainment talent who were not to receive government funding. She also leaned on CJ to make patriotic movies such as Roaring Currents and Ode to My Father.