Samsung chief Lee Jae-yong is being questioned at the prosecutor's office in Seoul as a suspect in the country's biggest political corruption scandal.
He told reporters outside he was "very sorry to the South Korean people".
The firm is accused of giving donations to several non-profit foundations operated by Choi Soon-sil, a confidante of President Park Geun-hye.
The donations were allegedly made in exchange for political support of a controversial merger.
Politicians voted on 9 December to impeach President Park over the scandal - a decision South Korea's constitutional court has six months to uphold or overturn.
Until then she remains formally president but stripped of her powers, which are handed to the prime minister, a presidential appointee.
Ms Choi is on trial for charges including corruption and coercion.
Earlier this week two other Samsung executives were interviewed by the special prosecutors, but were treated as witnesses rather than suspects.
The claims against the company circle around a merger between the electronics giant's construction arm, Samsung C&T, and an affiliate firm, Cheil Industries.
Prosecutors allege that Samsung gave €2.8 million ($NZ4.2m) to a company co-owned by Ms Choi and her daughter, in return for Ms Park's support for the deal.
Lee Jae-yong has already given evidence to politicians over the scandal, but this is the first time he will be quizzed as a suspect by investigators.
At the parliamentary hearing in December Samsung admitted giving a total of 20.4bn won ($NZ24.4m) to the two foundations, but denied seeking favours.
And Mr Lee also confirmed the firm gave a horse and money to help the equestrian career of Ms Choi's daughter, Chung Yoo-ra, something he said he now regretted.
Mr Lee is currently vice-chairman of Samsung Electronics. But since his father, Lee Kun-hee, suffered a heart attack in 2014, he is considered de facto boss of the entire Samsung Group conglomerate.
Ms Park's position began to unravel in October last year when details of her friendship with Ms Choi began to emerge.
They included revelations that the president had allowed her old friend - who holds no government role - to edit political speeches.
Since then, hundreds of thousands of protesters have gathered every weekend in Seoul to demand Ms Park stands down.
Ms Park denies wrongdoing but has apologised for the way she managed her relationship with Ms Choi, who also denies committing criminal offences.