Who owns the house? It’s theirs!
Theirs is the correct possessive pronoun of their. Their’s is incorrect and a mistake in English.
Let’s look at some more examples and then learn more about the grammar.
That dog is not ours, it’s theirs!!
I am sure that book is not yours, It is theirs
When do we use theirs?
Theirs is known as a possessive pronoun in English grammar.
A pronoun replaces a noun in a sentence. We use this pronoun because the noun is understood by the other person and it saves us from repeating the same noun over and over again.
Their is a possessive adjective in English. Their helps us to understand who owns something. In this case, it is the third person plural “they”
So..
We use theirs when we want to show that a group of people own something and we don’t want to repeat the noun.
In this example:
That dog is not ours, it’s theirs!!
You can also say:
That dog is not ours, it’s their dog!!
But we know that we are talking about a dog so there is no need to repeat dog!
Here is a table to show you the different Possessive adjectives and Possessive pronouns.