Amjad Jalal Ahmed Al-Dahan wore a fake beard and a cardboard belt with two empty water bottles taped to it.
He also had on a white robe with a scarf wrapped around his head at the party in an apartment complex in Petaling Jaya, a city next to Kuala Lumpur.
Al-Dahan, 34, pleaded guilty and was fined USD 100, the maximum fine for the offence, his lawyer Saraswathy Devi told AFP.
Largely Muslim Malaysia has arrested hundreds of suspected militants in recent years, including several people connected to the Abu Sayyaf group operating in the southern Philippines.
The Philippine government and security analysts said the attack on Marawi is part of an IS plan to establish a caliphate in Southeast Asia.