The bank said it has few gun manufacturing companies as clients, but those it does work with will be asked to provide details about their product and distribution networks.

If business customers decline Citigroup’s restrictions, the bank said it would work with them to “transition their business away.” The company declined to name clients or describe the extent of affected partnerships but said that “real revenue is at risk” if relationships fall through and customers protest. Some of Citigroup’s clients, like Walmart, already adhere to the new policies.

Citigroup said it did not have the technology nor the legal ability to monitor gun purchases at the payment-processing level, but said that the industry was discussing the possibility.

Edward Skyler, an executive vice president at Citigroup who helped craft the policy, wrote in a blog post that the company’s announcement “will invite passion on both sides.” But he stressed that the policies were “not centered on an ideological mission to rid the world of firearms.”

Following various mass shootings, “we have waited for our grief to turn into action and see our nation adopt common-sense measures that would help prevent firearms from getting into the wrong hands,” he wrote. “That action has sadly never come and as the weeks pass from the most recent mass shooting, it appears we remain in the same cycle of tragedy and inaction.”

The company said it would not enforce a specific deadline and instead hoped to have all of its clients on board with the gun restrictions within a few months. Members of its corporate citizenship group, which also monitors whether clients engaged in coal mining and oil drilling are adhering to Citigroup’s environmental standards, will check in regularly on partners’ gun-selling procedures, the company said.

The Citigroup board was supportive of the decision. Mr. Corbat said that, in many recent discussions about the new policy, “there hasn’t been a consensus, but we also haven’t come to this decision in a vacuum.”