The 38-year-old, who quit A-League team Melbourne City in December, had been seeking a new club to boost his chances of playing for Australia at a fourth successive World Cup.
Australia's all-time top scorer with 50 goals, Cahill left Melbourne after netting 13 goals for the Manchester City-owned club in all competitions since joining in August 2016.
He scored twice to help Australia beat Syria in the Asian World Cup playoff in October before they beat Honduras in the intercontinental playoff to reach this year's finals in Russia.
Cahill, who played for Premier League Everton and had stints in the United States and China, scored 57 goals for Millwall in 251 games from 1998 to 2004 to become a fans' favourite.
(Writing by Ken Ferris; Editing by Pritha Sarkar)
