It wasn't clear whether the president was basing his tweet on direct knowledge that he is under investigation, or on reports this week that special counsel Robert Mueller is examining whether the president obstructed justice by firing Comey last month amid the ongoing Russian Federation investigation.
The move suggested Mueller is examining whether Trump sought their help in trying to get former FBI chief James Comey to end an investigation into Michael T. Flynn, the former National Security Adviser.
Last week, the ranking Democrat on the House intelligence committee, Rep. Adam Schiff of California, said the committee would ask Johnson to testify about interactions former President Barack Obama's administration had with secretaries of state and local election officials about the dangers the Russian hacking posed to election systems.
In response to the news, Mark Corallo, spokesman for Trump's personal lawyer, Marc Kasowitz, said in a statement, "The FBI leak of information regarding the president is outrageous, inexcusable, and illegal".
Mr Comey said he was "sure" Mr Mueller was looking at whether Mr Trump had obstructed justice, but added that to his knowledge, the president had not tried to impede the Russian Federation inquiry.
Investigators were also looking for any evidence of possible financial crimes among Trump associates.
Trump said he was being investigated by the man who, in the first place, told him to fire James Comey. It's possible that Rosenstein ordered the obstruction probe before Mueller's appointment in May.
Mr Comey, who had been leading one of several Russian Federation inquiries, testified to Congress last week that the president had pressured him to drop an inquiry into links between Mr Flynn and Russian Federation.
According to the Post, Trump's firing of Comey on May 9 launched an investigation weeks before the former FBI director took the stand.
Trump hadn't publicly admitted he was personally under investigation until now.
Trump, in a series of morning tweets, reverted back to the tone that won him the White House, slamming the investigation a day after he called for unity in the wake of a shooting at a Republican congressional baseball practice that injured several people, including House Majority Whip Steve Scalise.
Before Trump took to Twitter on Friday, Rosenstein on Thursday issued a perplexing statement of his own, denouncing news stories based on anonymous sources.