Siege warfare and its devastating humanitarian consequences have been one of the defining features of contemporary armed conflicts. While the most apparent restriction of siege warfare appears to be provided by the prohibition against starvation of the civilian population as a method of warfare, the prevailing restrictive interpretation of this prohibition has left civilians remaining in a besieged area unprotected from the hardships they endure. This article demonstrates that shifting the focus from the prohibition against starvation to the rules regulating humanitarian relief operations does not seem helpful due to the ambiguities regarding the requirement of consent and the right of control of the besieging party. In remedying this protection gap, this article examines whether and how the principle of proportionality applies in the context of a siege. After analyzing whether the encirclement and isolation aspect of a siege can be considered an attack in the sense of Article 49(1) of Additional Protocol I (AP I), to which the proportionality principle applies, the article investigates how this principle operates in the context of a siege. It will be demonstrated that Article 57(2)(b) of AP I requires that the proportionality of a siege must be continuously monitored.