The 24-year-old midfielder has penned a two-year deal with the Blues, according to Tasnim News Agency.
Koushki is Esteghlal’s fifth signing in the transfer window.
Esteghlal has previously completed the signing of Milad Zakipour, Mohammad Hossein Eslami, Ramin Rezaeian and Saman Fallah.
Also on Saturday, Farshid Samiei was named as Esteghlal football club’s general manager.
Samiei was working as the club’s acting president since parting ways with Ali Khatir on May 1.
He is a lawyer and has nearly two decades of experience in the role.
Under his leadership, Esteghlal has completed signing of Milad Zakipour, Saman Fallah, and Ramin Rezaeian so far.