During the presidential campaign, Donald J. Trump pointed to his relationship with Deutsche Bank to counter reports that big banks were skeptical of doing business with him.

After a string of bankruptcies in his casino and hotel businesses in the 1990s, Mr. Trump became somewhat of an outsider on Wall Street, leaving the giant German bank among the few major financial institutions willing to lend him money.

Now that two-decades-long relationship is coming under scrutiny.

Banking regulators are reviewing hundreds of millions of dollars in loans made to Mr. Trump’s businesses through Deutsche Bank’s private wealth management unit, which caters to an ultrarich clientele, according to three people briefed on the review who were not authorized to speak publicly. The regulators want to know if the loans might expose the bank to heightened risks.