The Justice Department is seeking to reach a plea deal in its criminal investigation of the former son-in-law of Paul Manafort, President Donald Trump’s one-time campaign chairman, according to people familiar with the matter.

The investigation into Jeffrey Yohai—who hasn’t been charged with any crime—by the Federal Bureau of Investigation and U.S. attorney’s office in Los Angeles is separate from the Washington-based probe of his former father-in-law and others by Special Counsel Robert Mueller, who is examining Russian influence...