BEIJING—President Xi Jinping’s two-year antigraft campaign is hitting China’s vast financial sector, according to officials with knowledge of the matter, after investigators began questioning a senior executive at a major bank over his political ties. Mao Xiaofeng, until recently a rising star at China Minsheng BankingCorp. , resigned as president for “personal reasons,” Minsheng said on Saturday. Chinese anticorruption officials are questioning Mr. Mao over his ties to a former top Chinese Communist Party official, Ling Jihua, who is himself being probed by graft inspectors, according to an official at one of China’s financial regulatory agencies.

Mr. Mao couldn’t be reached for comment. It isn’t clear whether he will face accusations of wrongdoing. In China, authorities have broad powers to detain people. Minsheng, China’s largest non-government-controlled bank by assets, didn’t respond to requests for additional comment. Earlier, it said its operations weren’t affected.

Mr. Ling, a onetime top aide to former President Hu Jintao, was put under investigation by the party’s discipline commission in December for unspecified violations of party discipline, according to a statement from the antigraft agency at the time. Such violations are a euphemism for corruption among Chinese officials,

Mr. Mao, a 42-year-old banker who has an M.A. from Harvard University’s Kennedy School of Government, is the highest-ranking Chinese banking executive to have been touched by the antigraft campaign.

The probe of Mr. Mao, which was reported over the weekend by Caixin, an influential Chinese magazine, represents a widening of President Xi’s assault on corruption since he took the helm of China’s Communist Party in late 2012. Dozens of officials in state-controlled industries ranging from oil to resources to railroads have been detained or charged over corruption allegations.

Now, investigators’ focus is shifting toward the financial industry, which  has fallen under official scrutiny in the past.

The Communist Party’s Central Discipline Commission, headed by President Xi’s top enforcer, Wang Qishan, recently has formed a new department to mainly focus on the financial sector, according to the Chinese officials familiar with the matter. “Finance is Wang Qishan’s old stomping ground,” said one of the officials, referring to Mr. Wang’s experiences cleaning up the country’s debt-laden lending sectors in the late 1990s. “He knows how the industry works.”

Officials at the discipline commission didn’t respond to requests for comment.

President Xi is also stepping up the party’s oversight over the watchdogs responsible for policing the country’s banking, securities and insurance sectors—all of which have grown in size and complexity in recent years. The Communist Party’s secretive Central Policy Research Office, led by President Xi’s top political adviser, Wang Huning, recently has started to “give instructions” to the country’s three top financial regulators, according a party official with direct knowledge of the matter.

Those agencies—the China Banking Regulatory Commission, the China Securities Regulatory Commission and the China Insurance Regulatory Commission—generally answer only to the State Council, China’s cabinet. Press officials at those agencies didn’t respond to requests for comment.

The move is aimed at improving coordination among the three regulators as the industries they oversee become increasingly intertwined, the party official said. In recent years, as Beijing pushes for greater financial liberalization, many of the country’s banks have branched out into brokerage and insurance, while insurers have increasingly expanded their businesses into lending.

Minsheng was founded in 1996 by Jing Shuping, a prominent Chinese lawyer and businessman, as the first bank in the country that wasn’t controlled by the government. In the following years, the bank has also built up a sizable brokerage business.

In recent weeks, China’s Anbang Insurance Group Co. has increased its stake in Minsheng to more than 22% from 5%, becoming the company’s single largest shareholder. The sharp increase in its stake is leading to market speculation that the insurer would take control of Minsheng, as Mr. Mao’s resignation might lead to a shake-up of the bank’s management team. Minsheng’s Hong Kong-traded shares plunged as much as 10% early Monday morning before rebounding to end the day down 3%.

In a statement Monday, Minsheng said Anbang’s investment is “purely financial” and reflects the insurer’s “positive outlook” for the bank. Anbang, which attracted attention recently for its $1.95 billion purchase of the Waldorf Astoria hotel in New York, is known within the country for its political connection: It counts Chen Xiaolu, son of a leading revolutionary figure, as a board member and adviser.

Attempts to reach Mr. Chen and press officials at Anbang weren’t successful. Mr. Chen told local media last week that he has no stake in Anbang and doesn’t receive any compensation from the company.

With a market value bigger than Deutsche Bank AG’s, Minsheng has been one of the fastest-growing banks in China in recent years. The bank’s latest financial statements show it generated $7.3 billion in profit for the 12 months through Sept. 30, while bad loans accounted for 1% of its total lending. But some analysts are concerned that the bank’s loans to small businesses are prone to default amid a slowing Chinese economy.