Iacovitti, 18, made his senior debut for Forest in their 4-3 opening day victory over Burton Albion.
Stags manager Adam Murray bolstered his defensive options after Kyle Howkins suffered an hamstring injury in Tuesday's loss to Doncaster Rovers.
"He's an old-school centre half who can play and has good pace," Murray told the club website.
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Mansfield Town have signed Nottingham Forest defender Alex Iacovitti on a season-long loan.