The 33-year-old Kenyan international has this evening agreed a deal until the end of the season, subject to international clearance, and could make his debut for the club when they visit Ayr United in the SPFL Championship on Saturday.
He becomes new manager Neil Lennon's first addition since he was appointed as the new Pars manager last Friday, and it will mean the pair will be reunited having previously worked together at Celtic Park.
Reports began to surface earlier today that Wanyama was set to join Athletic, and was undergoing a medical, and the club have tonight confirmed that they have secured his signature.
Capped 64 times by his country, defensive midfielder Wanyama began his career in Belgium with Beerschot, before Lennon brought him to the east end of Glasgow in 2011.
During a two-year stint with the Scottish Premiership champions, he made 91 appearances across all competitions, scoring 13 goals, the second of which came in a 3-0 win over the Pars in an SPL match at East End Park.
After winning two top flight titles and the Scottish Cup, Wanyama left the Hoops to join Southampton for a fee of £12.5 million - which was, at the time, a record Scottish sale.
He became the first Kenyan to play in the English Premier League and, after three years at St Mary's, he joined Tottenham for a reported £11 million.
Wanyama penned a five-year contract with the North London side, signing for Maurico Pochettino for a second time, after the Argentine boss had taken him to Southampton.
He played 97 times for both Saints and Spurs, and was on the bench for the latter when they contested the 2019 UEFA Champions League final, which they lost 2-0 to Liverpool in Madrid.
In March 2020, Wanyama left Tottenham for MLS (Major League Soccer) side CF Montreal, playing over 140 times between 2020 and 2024.
He became a free agent on January 1 this year, as his Montreal contract extension came to an end, with his last appearance for the club coming in July 2024.