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 United States than in other countries. This prevalence of will contests in the U.S. is partly because the 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.

Standing to contest will.

Typically, standing in the United States to contest the validity of a will is limited to two classes of persons:

Those who are named on the face of the will (any beneficiary);

Those who would inherit from the testator if the will was invalid.

For example, Monica makes a will leaving $5,000 each to her husband, Chandler; her brother, Ross; her neighbor, Joey and her best friend, Rachel. Chandler tells Monica that he will divorce her if she does not disown Ross, which would humiliate her. Later, Ross tells Monica (untruthfully) that Chandler is having an affair with Phoebe, which Monica believes. Distraught, Monica rewrites her will, disowning both Chandler and Ross. The attorney who drafts the will accidentally writes the gift to Rachel as $500 instead of $5,000 and also accidentally leaves Joey out entirely.

Under such facts:

Chandler can contest the will as the product of fraud in the inducement, because if the will is invalid, he will inherit Monica's property, as the surviving spouse.

Ross can contest the will as the product of Chandler's undue influence, as Ross will inherit Monica's property if Chandler's behavior disqualifies Chandler from inheriting (however, many jurisdictions do not consider a threat of divorce to be undue influence).

Rachel has standing to contest the will, as she is named in the document, but she will not be permitted to submit any evidence as to the mistake because it is not an ambiguous term. Instead, she will have to sue Monica's lawyer for legal malpractice to recover the difference.

Finally, neither Joey nor Phoebe is someone who stands to inherit from Monica, nor is either named in the will, and so both are barred from contesting the will altogether.