The plural of thief is thieves.
A thief is a person who steals from others, usually in secret.
Most English nouns ending in -f have an irregular plural formed by replacing the f with -ves. The word thief follows this same rule and so the plural of thief is thieves.
Singular Example: The thief was caught after leaving behind his fingerprints on the door.
Plural Example: The thieves were apprehended before they could even enter the building.
The production team on the classic film 'Home Alone' originally tried to have Robert De Niro play the character of one of the thieves.