The 35-year-old is the latest addition to the backroom staff following the arrival of Scott Brown as head coach.
Fon Williams began his career at Crewe Alexandra before moving to Stockport County in 2008, impressing during his time with the Hatters.
He also had a loan move to Bury before a short spell with Rochdale, followed by a four-year stay at Tranmere Rovers.
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Moves to Inverness Caledonian Thistle, Hamilton Academical and Dunfermline Athletic followed for Fon Williams, who made his Wales debut against the Netherlands in 2015 and was part of their Euro 2016 squad.
Fon Williams told the club website: “I am delighted to be here at Fleetwood Town and I just can’t wait to hit the ground running.
“It’s an exciting time to be here, working alongside the new management team and, of cours,e a very talented group of goalkeepers – who have impressed me so far this pre-season.
“I hope I can pass on my experiences to this group, and like the rest of the coaching staff, we can’t wait for the season to start.”
Town have also confirmed that tickets are now on sale for their final pre-season match against Dundee United.
The game at Highbury, on Saturday, July 23, is the only chance to see Brown’s players on home soil ahead of the new League One campaign.
Tickets cost £12 for adults, £5 for concessions and £1 for under-18s with Onward Card holders having until 10am next Monday, July 11, to reserve their seats.