Abreu, 44, made his senior debut with Uruguay's Defensor Sporting in 1994 and set a Guinness world record for the most professional clubs played for when he joined Chilean side Audax Italiano in 2017 to mark 26.
"How do I feel physically? Let the ball roll and see," Abreu said during his unveiling. "The commitment and will is there. If I am on the field it is because I have the conditions, but the most important thing was what the coach said when he did a scout evaluation, looking at the 90-minute tapes and seeing that I can help Athletic."
South America's football governing body, CONMEBOL, congratulated Abreu, on Twitter and wrote: "Sebastian "El Loco" Abreu definitely thinks big and breaks his own record, now at 44 signing for his 30th team Athletic Club de Minas Gerais."
Athletic Club MG were recently promoted to the First Division of the Mineiro Championship after a 51-year wait.
Abreu, who had not seen action since last year, when he was on the books of Uruguay outfit Boston River as a player-coach, decided to return to the pitch after Athletic firmly expressed their desire to sign him.
"They had already wanted me last year but it didn't work out," Abreu said. "When the club was promoted for the first time in 51 years to play in the first division of the state competition, they called me and said that they still had the same desire to want to bring me in. This is fundamental because I do not underestimate my conditions, but when I see people who believe as I believe [in myself], it was essential for me to be here today."
It is Abreu's fifth experience in Brazil following his previous spells at Botafogo, Figueirense, Bangu and Rio Branco.
Abreu has also represented Uruguay in two World Cups and helped his country to the 2011 Copa America title.