The 26-year-old attacker, who only signed on a two-and-a-half-year deal at The Valley in January, has joined Kuwait Sporting Club on a season-long loan.
Charlton head coach Bob Peeters said: “It was Reza's decision to leave. He told us that, after the World Cup, he wanted to go to another country.
“If a player wants to leave then it makes it difficult because their mind is already weak from wanting a transfer – that is why we made the decision to allow Reza to leave.
"Because he has left we may now look to see if we can bring in a replacement and see who is available, but at this moment we are working with the squad we have and I am happy with them."
Ghoochannejhad, who became Charlton’s first ever player to score in a World Cup when he tapped in a late consolation in the 3-1 loss to Bosnia & Herzegovina, scored just once for the Addicks in 17 appearances.
Charlton have so far signed nine players in the current transfer window, including former Premier League defenders Tal Ben Haim and Andre Bikey. They have also captured midfield player Frank Moussa from Coventry.
They suffered a blow earlier in the summer, however, when exciting youngster Diego Poyet opted to move to West Ham.
Charlton begin their Championship campaign on Saturday with a trip to newly-promoted Brentford at 3pm.