What’s the difference between theirs and their’s?
Theirs is the third person plural possessive pronoun – it replaces their + noun.
Though you may see their’s written even by native speakers, it is incorrect. Theirs should never have an apostrophe.
The Bottom Line
The idea that theirs needs an apostrophe comes out of the fact that on virtually every other word, ‘s indicates possession, so English speakers sometimes think theirs should be spelled their’s. However, this is always incorrect – theirs is the only correct spelling.