A Canadian Postnuptial Agreement, legally known as a marriage contract in most provinces, is a domestic contract entered into by two people who are already married to each other. Unlike a prenuptial agreement which is signed before the wedding, a postnuptial agreement is executed during the marriage to establish the spouses' respective rights and obligations regarding property division, spousal support, financial matters, and other issues that may arise during the marriage or upon separation or divorce.

In Ontario, the legal authority for postnuptial agreements is found in Section 52(1) of the Family Law Act (R.S.O. 1990, c. F.3), which provides that two persons who are married to each other may enter into an agreement in which they agree on their respective rights and obligations under the marriage or on separation, on the annulment or dissolution of the marriage, or on death. The formal requirements are set out in Section 55(1): the contract must be in writing, signed by both parties, and witnessed. Courts apply heightened scrutiny to postnuptial agreements compared to prenuptial agreements, as established by the Supreme Court of Canada in Hartshorne v. Hartshorne (2004 SCC 22), particularly examining whether both spouses had independent legal advice, whether full financial disclosure was provided, and whether the agreement operates fairly at the time of distribution.

In British Columbia, postnuptial agreements are governed by the Family Law Act (S.B.C. 2011, c. 25), which allows spouses to contract out of the default property division regime. In Quebec, a marriage contract must be executed before a notary in authentic form under Article 440 of the Civil Code of Quebec, and changes to an existing contract also require notarial intervention.

A postnuptial agreement is needed when married couples wish to establish or modify the financial terms of their marriage after the wedding has taken place. This situation commonly arises when one spouse receives a significant inheritance or gift during the marriage and wants to protect it from equalization claims. Under Ontario's Family Law Act, gifts and inheritances are excluded from net family property calculation, but the exclusion can be lost if the property is commingled with family assets or used to acquire the matrimonial home.

Couples may need a postnuptial agreement when one spouse starts a business during the marriage and both parties wish to establish how the business value will be treated upon separation. The agreement can specify that the business, its goodwill, and future appreciation remain with the founding spouse, or that both spouses share in the business growth. This is particularly important in Canada where business valuation disputes are among the most contentious aspects of family law proceedings.

A postnuptial agreement is also essential after a significant change in financial circumstances, such as a major career change, retirement, or when one spouse assumes the role of primary caregiver. Couples who married without a prenuptial agreement and later realize they need to establish property division terms should execute a postnuptial agreement. Additionally, when couples reconcile after a period of separation, a postnuptial agreement can replace an earlier separation agreement and establish new terms for the ongoing marriage.

A valid Canadian postnuptial agreement must identify both spouses with their full legal names, addresses, and dates of birth, along with the date and place of marriage. The agreement must explicitly state that the parties are married and not contemplating separation, as a court will scrutinize the circumstances under which the agreement was made. Full and complete financial disclosure by both spouses is absolutely essential. Under Ontario FLA s. 56(4)(a), a court may set aside the entire agreement if a party failed to disclose significant assets or debts. Both spouses should provide detailed schedules of their assets, debts, income, and liabilities with approximate values in Canadian dollars.

The property division provisions must address how net family property will be divided upon separation or divorce. The agreement should identify excluded property, including gifts, inheritances, and pre-marriage assets, and specify whether income or appreciation from excluded property remains excluded. The matrimonial home requires special treatment: under Ontario FLA s. 52(2), a marriage contract cannot limit a spouse's right to equal possession of the matrimonial home, though it can address the home's value in equalization calculations.

Spousal support provisions should clearly state whether support is waived, agreed upon according to the Spousal Support Advisory Guidelines, or subject to custom terms. Courts retain jurisdiction under the Divorce Act to override support waivers that produce unconscionable results. Debt allocation provisions should specify responsibility for pre-marriage and marital debts. The agreement must include witness signatures (required in Ontario under s. 55(1)), and should include certificates of independent legal advice from each spouse's lawyer to maximize enforceability.