The Australian legend is flying into London this weekend to complete a move back to Millwall, according to the South London Press.
Cahill kicked off his professional career in England with the Lions in 1998 and played a key role in the club's remarkable run to the FA Cup final in 2004.
He also won the League One title and played in two play-off finals in his six-year spell at the south London club.
Cahill went on to join Everton in the Premier League, where he became a big fans' favourite.