Iran lost to Russia 3-2 (25-21, 19-25, 25-23, 21-25, 14-16) in Pool C.
Maksim Sapozhkov led the Russian team with 34 points and Pouria Yali had 30 points for Iran.
Iran, who started the campaign with a 3-0 win over Tunisia, will meet Czech Republic on Saturday.
The 2019 FIVB Volleyball Men's U-21 World Championship is being held in Riffa, Bahrain from 18 to 27 July 2019.
A total of 16 teams from the 5 confederations compete in the tournament.