Jus tertii (English: rights of a third party/ stranger) is a term for the legal argument by which a person can defend a claim made against them by invoking the rights of a stranger to the dispute. The defence asserts that the rights of the stranger are superior to those of the claimant; in other words the defence is that the claimant has insufficient right to make the claim he does, and that the stranger is the one to whom the defendant is really answerable.
Generally the argument is an attempt by the defendant to justify their own entitlement to possession by showing that legal title belongs to a person other than the person with actual possession. By showing legitimate title belongs to a third party (tertius), jus tertii arguments imply that the present possessor’s interest is illegitimate or that the present possessor is a thief.
In English law the argument is also sometimes used to defend against a claim by a person with possession of a thing, on the grounds that the claimant is not the true owner. (See the Torts (Interference with Goods) Act 1977 s 8(1).). This is unusual, and in most cases the defence of jus tertii is considered ineffective: a claimant with possession is, as regards a wrongdoer, entitled to the full rights of an owner; and this is so even if, as regards the stranger, his title is less than that (The Winkfield [1902] P 42).
Under United States law, jus tertii arguments are generally insufficient to support actions for replevin because they fail to show that possession is more legitimate in the third party than in the present possessor.[1] However, a bailee or other legal agent of the owner may successfully assert the argument.
The principle is sometimes used to allow one person to enforce or test the constitutional rights of another, which usually can't be done due to lack of standing. This is only possible for fundamental rights, where there is a close connection between the person whose rights are violated and the person wishing to enforce them, and the constitutional right being enforced is a fundamental right. See, e.g., Singleton v. Wulff.[2]