WASHINGTON—President Donald Trump said Thursday that an FBI agent once involved in the special counsel’s Russia probe committed “treason,” and he called for Republican investigators in Congress to conclude their probes swiftly.

In an interview with The Wall Street Journal, Mr. Trump, unprompted, labeled as “treason” a text message written by Peter Strzok, an agent at the Federal Bureau of Investigation who was removed in July as a top investigator on the special counsel’s team investigating whether Russia colluded with associates...