WASHINGTON—Gordon Sondland, the U.S. ambassador to the European Union, said Wednesday at a public hearing that “at the express direction” of President Trump he urged Ukraine to announce investigations that would benefit the president politically.

Mr. Sondland testified there was a “quid pro quo” between a White House meeting for the new Ukrainian president and the investigations Mr. Trump sought, and explained in detail why he believed nearly $400 million in aid to Ukraine had been linked to those probes. Democrats said his...