US Deputy Attorney General Rod Rosenstein has handed in his resignation to President Donald Trump.
The two men had a rocky relationship. In 2017, Mr Rosenstein appointed Special Counsel Robert Mueller to probe claims of Russian interference in the 2016 presidential election.
In a resignation letter, he wrote: “We enforce the law without fear or favour because credible evidence is not partisan.”
He will leave the post on 11 May.
Correspondents say Mr Rosenstein had been expected to resign in March, following the appointment of Attorney General William Barr.
However, he stayed in the job longer to help Mr Barr manage the public release of the special counsel’s findings from the investigation, which has dominated much of Mr Trump’s presidency so far.
Mr Trump has repeatedly called the investigation a “witch hunt”.