Allen Weisselberg, a longtime financial gatekeeper for President Donald Trump, has been subpoenaed to testify before a federal grand jury in the criminal probe of Mr. Trump’s former personal lawyer, Michael Cohen, according to people familiar with the investigation.

Mr. Weisselberg is considered a witness in the investigation, the people said. It isn’t known whether he has already appeared before the grand jury or what questions prosecutors of New York’s Southern District have had for him. The date of the subpoena couldn’t...