A $100,000 real-estate brokerage fee that was part of former Trump attorney Michael Cohen’s guilty plea Tuesday came from representing a company owned by a member of the Qatar royal family, according to interviews and real-estate documents.

Mr. Cohen admitted to failing to pay taxes on more than $4 million in income, among other felonies. That income included what prosecutors described as $100,000 in 2014 from “brokering the sale of a piece of property in a private aviation community in Ocala, Florida.”

The community is a fly-in development named Jumbolair, said Frank Merschman, who owns the development. Mr. Merschman said he bought some property in 2014 from a Qatari company, and Mr. Cohen represented the seller, Abdul Aziz Althani Holdings Inc.

That company is owned by a member of the Qatar royal family, Sheikh Abdul Aziz bin Jassim bin Hamad Al-Thani, 62 years old, according to the Panama Papers, documents from a Panamanian law firm obtained by the International Consortium of Investigative Journalists and the German newspaper Süddeutsche Zeitung, and shared with The Wall Street Journal.

Sheikh Abdul Aziz and his attorneys didn’t respond to requests for comment. He was described in the Panama Papers as a major shareholder in two Qatari banks.