The agreement was signed by Croatia Airlines general director Ivan Misetic, Star Alliance president Jaan Albrecht, and Lufthansa CEO Wolfgang Mayrkuber.
Slovenia's air carrier Adria Airways today also signed an agreement on joining the Star Alliance.
Both national flag carriers will become regional members of the Star Alliance on December 15. The Star Alliance is a global air transport association whose members offer more than 14,000 daily flights to 755 destinations in 132 countries.
Croatia Airlines representatives said admission to the Star Alliance represented the realisation of one of the company's strategic goals.
The Star Alliance was set up in 1997 as the first global air transport association which flies its passengers all around the world.