A will contest, in the law of property, is a formal objection raised against the validity of a will, based on the contention that the will does not reflect the actual intent of the testator (the party who made the will) or that the will is otherwise invalid. Will contests generally focus on the assertion that the testator lacked testamentary capacity, was operating under an insane delusion, or was subject to undue influence or fraud. A will may be challenged in its entirety or in part.

Courts and legislation generally feel a strong obligation to uphold the final wishes of a testator, and, without compelling evidence to the contrary, "the law presumes that a will is valid and accurately reflects the wishes of the person who wrote it".

A will may include an in terrorem clause, with language along the lines of "any person who contests this will shall forfeit his legacy", which operates to disinherit any person who challenges the validity of the will. Such no-contest clauses are permitted under the Uniform Probate Code, which most American states follow at least in part. However, since the clause is within the will itself, a successful challenge to the will renders the clause meaningless. Many states consider such clauses void as a matter of public policy or valid only if a will is contested without probable cause.

This article mainly discusses American law and cases. Will contests are more common in the US States than in other countries. This prevalence of will contests in the US is partly because American law gives people a large degree of freedom in disposing of their property and also because "a number of incentives for suing exist in American law outside of the merits of the litigation itself". Most other legal traditions enforce some type of forced heirship, requiring that a testator leave at least some assets to their family, particularly the spouse and children.