Woking confirmed the appointment on Sunday, with the club sat firmly in midtable in 11th place, on 53 points from their 39 games.
Defoe said: “Woking is a historic club with huge potential, and I’m thrilled to be part of this exciting project. I can’t wait to get started.”
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Club chairman Todd Johnson said: “Jermain’s achievements as a player speak for themselves, but what stood out to us during the process was how he sees the game, how he drives standards, and his approach to leadership and player development.
“We have a clear plan for where we want to go as a club, and we believe Jermain is the right person to build on the strong foundations already in place and help take us forward in the next phase of that journey.”
Director of football Jody Brown added: “Jermain’s love of the game, his determination to succeed and his alignment with our preferred playing philosophy were important.
“These are shared values in terms of the working environment, player development, and high-performance culture.
“He has a real appreciation for the club, the staff, the players, and what we’re building here.”
Defoe had two spells in Dorset with the Cherries, first on loan in the 2000/01 season and then on a permanent basis between 2017 and 2020.
He scored 23 goals in 64 appearances across those two stays at the club.
Defoe has brief experience as a head coach, as he was a part of a four-man caretaker team that took over at Rangers in Scotland when Steven Gerrard left the club for Aston Villa, while he was still a player in 2021.