Bardia Saadat scored 25 points for Iran and Georgi Tatarov led the Bulgarian team with 13 points.
Iran will play the winner of Slovenia and Japan on Friday for the 5th place match.
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.