Ifill, a 29-year-old British-born Barbados international, joins ex-Tranmere Rovers player Chris Greenacre in the Phoenix's attacking ranks as they look to overcome the loss of Shane Smeltz to the Gold Coast.
"There has always been a lot of pressure around signing a marquee-type player but we believed it was important to find the sort of player that was hungry to achieve and still had the passion to bring something to the club," Phoenix chief executive Tony Pignata said.
"In that respect we had very good reports regarding Paul and we are delighted to sign him. We wanted the right people and we're pleased we've got that.
"Paul has pace, height and international experience - all the qualities we were looking for. With Paul and Chris we've got very, very good attacking options."
Ifill will arrive in Wellington later next week to be considered for the Phoenix's opening A-League match against the Jets in Newcastle on August 9.