Pourya Yali led Iran with 15 points and Hasan Mohamed and Hasan Haider scored nine and seven points respectively for Bahrain.
Iran will meet Brazil Thursday night.
Italy will also play Russia in another semis.
The final will be held on July 27.
The 20th edition of the FIVB Volleyball Men’s U21 World Championship is taking place in Riffa, Bahrain.
