Is the correct spelling boss' or boss's?

Answer 1

Both spellings are correct.

There are two accepted forms for possessive singular nouns ending in s:

Add an apostrophe (') after the existing s at the end of the word: boss'

Add an apostrophe s ('s) after the existing s at the end of the word: boss's

Example:

Put the mail on the boss' desk.

Put the mail on the boss's desk.

Answer 2

Both "boss'" and "boss's" are correct, but "boss's" is more commonly used.