The 20-year-old, who gained his first senior international caps earlier this summer, arrives at Welford Road as an injury dispensation signing with Geoff Parling ruled out by a neck injury.
Mafi moves to the northern hemisphere from the Waratahs in Australia.
Leicester Tigers director of rugby Richard Cockerill said: “We’ve got good competition right through the squad but injuries to Richard Blaze and Geoff Parling have left us a bit thin in the second row. It is very close to the start of the season and I’m delighted that we’ve been able to bring in Steve.
“He has come through the ranks within Australian club rugby and has been very highly regarded there. He’s a big, strong guy, he’s young and ambitious, and we’re looking forward to seeing how he settles into his rugby here.”
Mafi was a member of the Australia Schools and Under-20s squads before making his senior international debut for Tonga against Samoa in June this year.
Richard Blaze has been sidelined with a foot injury, while Parling ended last season with a Premiership champion’s medal but then suffered a neck injury during his first England senior tour to Australia and New Zealand this summer.
Cockerill said: “It’s very unfortunate for Geoff because he had an excellent first season for us, but he’s working his way back from having surgery and we wish him a speedy recovery.”
Steve Mafi Factfile
Name Sitiveni ‘Steve’ Mafi
Born December 9, 1989
Height 2.00m/6ft 7in
Weight 112kg/17st 8lb
Previous clubs Parramatta, West Harbour, Waratahs
Honours Tonga (3 caps)