Bulgaria is part of the European Union. Nationals of several countries can enter Bulgaria for short-term stays without a visa, including Schengen countries, the USA, Australia, and Canada.
A full list of countries whose citizens, holders of ordinary passports, require/do not require visas to enter Bulgaria can be found here : https://www.mfa.bg/upload/711/26_06_2017_EN_VisaRegime_for_foreigners_ordinary_passports.pdf
For more information about obtaining a VISA for Bulgaria, please visit the following website: www.mfa.bg.
If a visa is required, be sure to apply at least a month before your visit.