Ex-President Lee Myung-bak appeared for questioning at the Seoul Central District Prosecutors' Office on Wednesday morning on a slew of corruption charges.
The verdict in her trial on charges of bribery and abuse of power is due next month, with prosecutors demanding 30 years in jail.
Lee, who was president from 2008 to 2013, has previously denounced the inquiry as "political revenge" and said Wednesday he hoped it would be the "last time in history" that a former South Korean head of state was summoned for questioning by prosecutors.
Lee says the investigation is politically motivated.
Almost all former presidents in South Korea, their family members and key aides were embroiled in scandals either just before they ended their terms or after they left office.
The move came about two weeks after prosecutors demanded a 30-year prison term for Lee's conservative successor, Park Geun-hye, over a separate bribery scandal that led to weeks of massive anti-government protests.
Lee, accompanied by his defence lawyer, was received by a senior prosecutor and given tea before being ushered into room 1001, where Park underwent a marathon 21.5 hour questioning past year.
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It's alleged that Lee accepted almost 10 million dollars' worth of bribes during his presidential campaign and while he was in office.
They were looking for possible evidence that Samsung provided financial support in litigation fees for an auto parts maker called DAS, run by Lee's family and allegedly controlled by him, the prosecutors' office said. "Lee is also denying pocketing slush funds and leaking state documents, saying he doesn't know anything (about those matters)".
The heir to parent Samsung Group, Jay Y. Lee, was freed last month with a suspended jail sentence on bribery and embezzlement charges linked to the Park corruption case.
The National Intelligence Service, the country's spy agency, is believed to have delivered secret operation fund to Lee's office at his behest through his closest aides, some of whom already admitted the allegation.
Wednesday's interrogation of Lee comes 358 days after ousted President Park appeared at the prosecution's office to be questioned on March 21 a year ago.
Lee is also suspected of dodging taxes through accounting fraud in the DAS and by owning real estate assets and bank deposits under borrowed names. The prosecutors have prepared about 120 pages of questions, and Lee is expected to spend hours for the interrogation.
Former South Korean president Lee Myung-bak was questioned over corruption Wednesday, the last of the country's living ex-leaders to be embroiled in a criminal inquiry.