Generally, a premarital agreement trumps a will rather than the other way around, but there are exceptions. Most state laws prohibit a spouse from disinheritance unless there is a specific agreement -- such as a prenup -- between husband and wife where each waives their right to inherit. The provisions of a will can trump a prenuptial agreement only if the intent of the will to revoke or modify the prenumptial agreement is clear.
Most states include “elective share” provisions in their estate laws. An elective share is the right of a surviving spouse to a certain portion of the value of the deceased’s estate, usually one-third to one-half. The surviving spouse can “elect” to take that portion rather than whatever was left to him in the deceased’s will. This is true even if the entire estate is bequeathed to others, such as the deceased’s children from a previous marriage or relationship. The executor of the will would be obligated to liquidate the assets given to the children in order to give the spouse his elective share. The only protection against this is if the spouse waives his right to an elective share in a premarital agreement. A premarital agreement alone can’t prevent an elective share -- it must be specifically stated in the agreement that the right is relinquished.
Identification of Property
An elective share is usually a percentage of the value of the deceased’s entire estate, including assets that would normally bypass probate, such as life insurance policies with a named beneficiary. However, a premarital agreement normally identifies assets that were brought into the marriage and these could be exempt from elective share statutes. They are not marital property, something a spouse would have a legal right to share. A premarital agreement can trump portions of a will by removing these assets from the estate before the estate is divided.
In some instances and to the benefit of the surviving spouse, a will can trump a prenup if the will makes a specific bequest even though the surviving spouse waived her right to receive anything. If a will states that a spouse is bequeathing item "X" to his wife despite the fact that he has no obligation to do so under the terms of their premarital agreement, the terms of the will generally prevail as long as that intention is clear.
Ideally, your premarital agreement will contain the same terms as your will. A premarital agreement can obligate both parties to make a will after the marriage that mirrors the same provisions. If the will is made prior to the marriage, at the same time as the premarital agreement is signed, generally a revised will or new will is necessary to acknowledge the marriage. In this case, neither the will nor the prenup trumps the other because they confirm the intentions of each other.