The League Two club posted pictures of the 37-year-old Premier League, La Liga and Champions League winner in their training kit alongside the message: "Who have we got here then?
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"It's only three-time Premier League, Champions League, La Liga and African Cup of Nations winner Yaya Toure.
"There he is look, in his Leyton Orient top at training this morning."
Toure made his name at Barcelona, whom he joined in 2007, and later became a club legend at City when he moved there in 2009, winning three Premier League titles, two League Cups and the FA Cup.
He left City in 2018 and joined Olympiakos but his time in Greece only lasted three months with his contract terminated by mutual agreement.
Since then he had a short spell in China with Qingdao Huanghai and reportedly came close to joining Brazilian side Botafogo.
He denied that he had retired from football in 2019, despite his agent claiming the midfielder called time on his playing career.