Legal Definitions - supplemental agreement

A supplemental agreement is an additional agreement made between two or more parties that adds to or modifies an existing agreement. It is also known as a side agreement.

For example, if two parties have a contract for the sale of a car, they may later agree to add a supplemental agreement that includes additional terms, such as a warranty or payment plan.

Another example is in international trade, where a side agreement may be negotiated to supplement a broader trade treaty. For instance, NAFTA contains no provisions about labor standards or environmental protection, but two side agreements about those areas were negotiated separately and designed to supplement NAFTA, making the treaty more attractive to the ratifying bodies.