Iranian President Hojat-ol-Islam Hassan Rouhani addressed the opening ceremony.
The three-day gathering aims to discuss different issues related to the Muslim world media.
220 members of the union from 36 countries have come to Tehran to attend the assembly whose motto is "Media of Awakening, Voice of Resistance, and Confronting Religious Sedition", IRNA reported.
Palestine, Syria, and religious sedition will be on the agenda of the participants in the gathering.
On the sidelines of the assembly, 20 countries, including Iran, Iraq, Kuwait, Lebanon, Egypt, Palestine, Syria, India, Italy, UAE, Yemen and Pakistan will present their productions in the Islamic Film Market.