Samsung chief Lee Jae-yong is being questioned at the prosecutor's office in Seoul as a suspect in South Korea's biggest political corruption scandal.
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.
The scandal has led to President Park being impeached last December.
Earlier this week two other Samsung executives were interviewed by the special prosecutors, but were treated as witnesses rather than suspects.
Prosecutors allege that Samsung gave €2.8m euros ($3.1m; £2.5m) to a company co-owned by Ms Choi and her daughter, in return for Ms Park's support for the deal.
Lee Jae-yong, also known as Jay Y. Lee, has already given evidence to politicians over the scandal, but this is the first time he has been quizzed as a suspect by investigators.
At the parliamentary hearing in December, Samsung admitted giving a total of 20.4bn won (£16m; $17.46m) 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.