According to Politico, his resignation was announced as a footnote in a document submitted on Saturday afternoon.
In the document, a district judge was urged not to extend a previous decision to block the publication of Smith's report on Trump. Whether the report will be released before Trump is sworn in as president on January 20 remains uncertain.
Jack Smith had previously announced plans to leave his position before the change of power. Donald Trump has repeatedly promised to fire him as soon as he takes office as president.
Smith led two federal investigations into Trump, one related to the storming of the Capitol building and one about Trump's handling of classified documents in Mar-a-Lago, Florida, after he had left the White House.
The investigations were dropped after Trump won the presidential election. Smith then made a formal request for the cases to be scrapped, based on the Justice Department's policy that prohibits prosecution of a sitting president.
