Iran beat its European rival 3-1 (25-21, 21-25, 25-22, 25-20) in Pool B.
Amir Hossein Toukhteh led Iran with 27 points and Denis Karyagin was Bulgaria’s top scorer with 21 points.
Iran is pitted against the Czech Republic, Italy, Colombia and Bulgaria.
Iran will meet the Czech Republic on Friday.
The 2019 FIVB Volleyball Boys' U-19 World Championship is the sixteenth edition of the FIVB Volleyball Boys' U-19 World Championship, contested by the men's national teams under the age of 19 of the members of the Fédération Internationale de Volleyball (FIVB), the sport's global governing body.
The tournament is being held in Tunisia from August 21 to 30.