The Australian international has signed a four-year deal with the Merseyside club in a deal believed to be worth around £2 million.
The move brings to an end a career at The Den that last seven years and which saw the midfielder score 57 goals in 249 appearances. During that time, Millwall have enjoyed one of the most successful periods in the club's history, culminating in last season's F.A. Cup Final appearance.
Cahill joined the club as a bright-eyed 16-year-old from Sydney United and was handed his debut by then manager Billy Bonds on the last day of the 1997/98 season. The following year, he helped take Millwall to Wembley when they reached the final of the AWS Shield competition and became an integral part of a side they won the Second Division Championship and reached the play-offs in The Lions' first season back in Division One after a five-year absence.
Injury robbed the club of the player's services in 2002/03 but last season Cahill returned to form in style and scored the goal that booked The Lions' place in the Cup Final and secure their participation in this season's UEFA Cup Final.
A free-scoring midfielder, Tim Cahill will be very difficult to replace.