The 35-year-old joins the npower League Two side as a full-time goalkeeping coach, while also providing backup to Fleetwood duo Scott Davies and Chris Maxwell.
Lucas, who joined Blues last July, had expressed a desire to expand his horizons into coaching and Lee Clark was prepared to allow him to join Graham Alexander's staff at Highbury Stadium.
The former Preston North End man made one senior appearance for Blues during the 5-1 Capital One Cup victory over Barnet at St. Andrew's back in August.
"This was an opportunity too good for me to turn down,” Lucas told Fleetwood's official website.
“I’ve had a taster with Graham Alexander and Chris Lucketti at Preston North End, working with their youth team and academies on my day off, and it’s something I have really enjoyed as I’ve got older.
“The chance to work with senior goalkeepers is something I’ve been itching to do, and to combine that with playing while I’m still fit and able is fantastic.”
Everybody at Blues wishes the goalkeeper all the very best in the next stage of his career.
Blues U21s' goalkeeper Nick Townsend has now been given first-team squad number 37 as a result of Lucas' departure.