Special Counsel Robert Mueller’s office has interviewed Deputy Attorney General Rod Rosenstein about President Donald Trump’s firing of former FBI Director James Comey, according to people familiar with the investigation.

The interview, which occurred in June or July, presents the unusual situation of investigators questioning the person directly overseeing their probe. Mr. Mueller’s office is investigating Russia’s alleged meddling in the 2016 election, whether any associates of Mr. Trump coordinated with Moscow’s efforts,...