The genitive case is a grammatical case that indicates a relationship, primarily one of possession, between the noun in the genitive case and another noun. In a more general sense, this genitive relationship may be thought of as one thing belonging to, being created from, or otherwise deriving from some other thing. (The relationship is usually expressed by the preposition of in English). The term possessive case refers to a case that is similar, though usually more restricted in usage, to the genitive.

Specific varieties of genitive relationships include:

• origin ("men of Rome")
• composition ("wheel of cheese")
• part of a mass ("a pound of beef")
• number of distinct items (Old English "féower manna"; literally, "four of men")
• relationship ("Janet's husband")
• subjectivity ("my leaving")
• objectivity ("the archduke's murder")
• description ("man of honor", "day of reckoning")
• inalienable possession ("my height", "his existence", "her long fingers")
• alienable possession ("his jacket", "my drink")

The last two relationships are the most commonly expressed by the genitive.

In some languages, nouns in the genitive case also agree in case with the nouns they modify (that is, the head noun is marked for two cases). This phenomenon is called suffixaufnahme.

One form in which genitive cases may be found is inclusio.

Many languages have a genitive case, including Arabic, Latin, Irish, Greek, German, Dutch, Russian, Finnish and Sanskrit. English does not have a proper genitive case, but a possessive ending, -'s (see below).

The English -'s ending

It is a common misconception that English nouns have a genitive case, marked by the possessive -'s ending. Linguists generally believe that English possessive is no longer a case at all, but has become a clitic, an independent particle which, however, is always written and pronounced as part of the preceding word. This can be shown by the following example: "The king of Sparta's wife was called Helen." If the English -'s were a genitive, then the wife would belong to Sparta; but the -'s attaches not to the word Sparta, but to the entire phrase the king of Sparta.

That is not to say that the English possessive did not have its origins as a genitive case; but it has developed into being a clitic instead. In Old English, a common singular genitive ending was -es. Instead, the apostrophe is replacing the 'e' from the Old English morphology.

The 18th century explanation that the apostrophe might replace a genitive pronoun, as in "the king's horse" being a shortened form of "the king, his horse", is erroneous. Indeed, it would be expected that plurals and feminine nouns would form possessives using '-r': "*The queen'r children" would be short for "the queen, her children." Since this is different from the plural, it would provide a useful distinction. The fact that that is not how English speakers form possessives shows that the above explanation is incorrect.

A few remnants of the genitive case do remain in Modern English in a few pronouns as whose, the genitive form of who; likewise, my/mine, his/hers/its, our/ours, their/theirs. See also Declension in English.

Genitive Latin nouns in astronomy

In astronomy, it is important to know the genitive form of the Latin names of constellations, because these are used along with letters of the Greek alphabet to name stars. For example, since the genitive of Gemini is Geminorum, the star Castor, brightest in the constellation Gemini, is named α Geminorum. For more details, see Bayer designation.