Renzi has written to EC President Jean Claude Juncker to make the nomination official, Italian media reported.
The vote on Catherine Ashton’s replacement was postponed until August 30th after EU leaders failed to reach an agreement during a summit in late July.
Renzi has lobbied hard on his 41-year-old foreign minister's behalf and expressed annoyance that the European Council summit had failed to settle on a candidate.
“For the role of High Representative for foreign policy, there is no candidate other than the one from Italy," he said.
Mogherini’s decision to make Russia her first overseas trip as foreign minister – and just a week ahead of the summit – met with a backlash.
About 11 EU states, including Poland and Lithuania, are reportedly against giving the role to Mogherini.
Critics also point to Italy’s backing for South Stream, a controversial gas pipeline in southern Europe which they say is designed to blackmail Ukraine.