A purpose trust is a type of trust which has no beneficiaries, but instead exists for advancing some non-charitable purpose of some kind. In most jurisdictions, such trusts are not enforceable outside of certain limited and anomalous exceptions, but some countries have enacted legislation specifically to promote the use of non-charitable purpose trusts. Trusts for charitable purposes are also technically purpose trusts, but they are usually referred to simply as charitable trusts. People referring to purpose trusts are usually taken to be referring to non-charitable purpose trusts.

Trusts which fail the test of charitable status usually fail as non-charitable purpose trusts, although there are certain historical exceptions to this, and some countries have modified the law in this regard by statute. The court will not usually validate non-charitable purpose trusts which fail by treating them as a power. In IRC v Broadway Cottages Trust [1955] Ch 20 the English Court of Appeal held: "I am not at liberty to validate this trust by treating it as a power. A valid power is not to be spelled out of an invalid trust."

The basis for the general prohibition against non-charitable purpose trusts is usually phrased on one or more of several specific grounds.

A trust is, at its root, an obligation; accordingly, "every [non-charitable] trust must have a definite object. There must be someone in whose favour the court can decree performance." With a charitable trust, this power of enforcement is usually vested in the Attorney General. Such conceptual objections were weakened by the decision of the House of Lords in McPhail v Doulton [1971] AC 424, where Lord Wilberforce expanded the class of valid discretionary trusts, including trusts where the beneficiaries could not be listed, but beneficiary status could be easily determined.

Where the objects of a trust are a purpose rather than an individual or individuals, there is much greater risk that a trust would not be enforceable due to lack of certainty. Cases such as Morice v Bishop of Durham (1804) 9 Ves Jr 399 and Re Astor [1952] Ch 534 re-affirm the court's disinclination to enforce trusts that are not specific and detailed. It is noteworthy that the common law exceptions to the general prohibition on purposes trusts tend to relate to specific and detailed matters, such as maintenance of a specific tomb, or caring for a particular animal.

Purpose trusts have been attacked conceptually on the basis that it would amount to the delegation of a testamentary power, although subsequent cases have cast doubt on the correctness of that reasoning.

Charitable purpose trusts are exempt from the rule against perpetuities. Private trusts are not. Accordingly, all non-charitable purposes trusts, to be valid, need to comply with the perpetuity rules in the relevant jurisdiction.

There are, nonetheless, several well recognised exceptions at common law where non-charitable purposes trusts will be upheld.