Testamentary capacity is the legal and mental ability of a person making a will (the testator) to make a legally binding disposition of their assets and to understand the nature and effect of the act of making a will—including disposing of their assets at death.

Proof of testamentary capacity is a required element of a will being admitted to probate and recognized by the court as having legal effect.

Adults are generally presumed to have the legal capacity to make a will, but minors (persons under a certain age) are not—with some exceptions in some states, such as if the minor has served in the military or has been married.

Persons who suffer from dementia, Alzheimer’s disease, or insanity, for example, may lack testamentary capacity to make a will.

In legal jargon, the historical terms of art for testamentary capacity were that the testator was “of sound mind and memory” or “of sound mind and disposing memory,” and this terminology is sometimes recited in a will as a statement of the testator regarding the testator’s mental capacity when making the will.

Laws vary from state to state and the requirements related to testamentary capacity are often located in a state’s statutes—usually in the estates code or probate code—and interpreted and applied by the courts in specific factual circumstances or cases that are reported in court opinions involving will contests, for example.

In Washington State, testamentary capacity is the legal and mental ability of an individual (the testator) to create a valid will. This includes understanding the nature of the act, the extent of their property, and the disposition of their assets upon death. Adults are presumed to have the capacity to make a will, while minors typically do not, unless they meet certain exceptions such as being married or having served in the military. Individuals with mental impairments, such as dementia or Alzheimer's disease, may lack the necessary capacity. Washington law requires that a person must be of 'sound mind' to make a will, which is a standard that can be challenged in probate court. The specific statutes governing testamentary capacity in Washington can be found in the Revised Code of Washington (RCW), particularly in the sections related to wills, trusts, and estate planning. If a will's validity is contested on the grounds of testamentary capacity, the courts will evaluate the evidence based on the circumstances of each case.