The 20-year-old will stay at St Mary's until the end of December and goes straight into George Burley's squad for the visit to Sheffield United on Saturday.
Ifil made his Spurs debut against Liverpool on the opening day of the 2004-05 season and has gone on to make three more appearances for the first-team.
He also enjoyed two successful loan spells with Millwall, and Saints boss Burley believes Ifil has a lot of potential.
"We have watched him a few times and I am delighted to be able to bring him in," he told the club's official website.
"Phil is a young defender with a lot of potential and he wants first-team football. It is a good chance to bring in a promising Premiership player and assess him."