The 24-year old Iranian international passed his medical examination on Wednesday in the Dubai-based club.
Madanchi, who has played for the Emirati Al-Shaab and Al-Nasr clubs, has been preferred to Brazilian Cesario in the club.
It is said that Madanchi joined Al-Ahli for 1 million euro.
Madanchi will accompany Al-Ahli in the 6th edition of the FIFA Club World Cup which will be hosted by Abu Dhabi in December.