In English grammar, possessive case is the case (or function) of an inflected form of a noun (Santa's, the boss's) showing ownership, measurement, or source. In addition to the -'s ending (a clitic), the possessive can be expressed with of, particularly when the possessor is not alive (the top floor of the building, the base of the statue).

The concept of case in English involves the relationship of a noun, a pronoun, or an adjective (also referred to as a determiner) with other parts of a sentence. The possessive case often conveys possession or ownership, such as Joseph’s book or my opinion. It is the only case in which nouns alter their form (e.g., Joseph to Joseph’s).

The genitive case is used to show possession or a relationship between nouns. It is typically indicated by the use of an apostrophe followed by an “s” (‘s) or just an apostrophe (‘) for singular nouns, and an apostrophe followed by an “s” (s’) for plural nouns. If the possession or the relation of noun is expressed, it is called Possessive Case.

The possessive case shows possession or ownership. For example: The dog has eaten the cat's dinner. (The apostrophe in "the cat's" tells us the cat owns the dinner.)