Having suffered a 3-1 defeat in the first leg in Jeddah last week which resulted in the sacking of coach Tomislav Ivic Al Ittihad produced a remarkable performance at the Seongnam Sports Complex to ruin the home side's hopes of salvaging a disappointing season.
Defender Rehda Tukar gave Al Ittihad the lead on 27 minutes when he rose above Park Woo-hyun to head in Chico's corner.
Hamza Idris made it 2-0 for the six-times Saudi champions in first-half injury time after Seongnam failed to clear a Chico freekick.
Seongnam were forced to go on the offensive in the second half which played into the hands of the counter-attacking Saudis coached by Dragan Talajic.
Al Ittihad took the aggregate lead when skipper Mohamed Noor tucked the ball past Seongnam keeper Yang Young-min after a run down the left by Manaf Abushgeer.
Noor made it 4-0 in the 78th minute and Abushgeer completed an amazing victory with a curling shot five minutes into injury time.
The defeat ended a miserable season for Seongnam who finished ninth out of 13 in the K-League having lifted the title the previous three years.
Al Ittihad's victory meant the Asian Champions League trophy stayed in west Asia as United Arab Emirates' Al Ain won the inaugural competition last year.