The league runners-up lifted the prestigious cup for the 12th time in their history with a comprehensive victory with Amad Al Hosni on the scoresheet
Karel Jarolim's side, who finished the 2011-12 Saudi Pro League campaign as runners-up, clinched the crown for the 12th time in its history.
The Royal Club went ahead on 34 minutes when Oman international Amad Al Hosni pounced on a rebound to open the scoring.
Abdulrahim Jaizawi added a second straight after the interval following Brazilian midfielder Marcelo Camacho's nice pass.
Muhammad Al Sahlawi pushed a header wide for Al Nasr, while Jaizawi came close again, before Haj Bugash was sent off in the 72 minutes, making the Riyadh club's task even harder.
Kamel Al Mousa added the third on 75 minutes and only a minute later Jaizawi made it four.
Al Nasr netted a late consolation through Brazilian Vinicius Cunha Reche, but Al Ahli were celebrating with King Abdullah, Custodian of the Two Holy Mosques, awarded the Royal Club the trophy along with the players their gold medals.