The Treaty on the Prohibition of Nuclear Weapons (TPNW) comprehensively and unequivocally prohibits nuclear weapons. The treaty was created to foster and diffuse norms against nuclear weapons, thereby stigmatizing and delegitimizing nuclear weapons and deterrence. The TPNW’s nature as formal treaty under international law suggests, however, that the TPNW primarily serves signaling to states which have not adhered to the treaty, in particular nuclear weapon states. This article develops how treaty law enables signaling to outsiders. Treaty law notably offers visibility, screens between “insiders” and “outsiders,” communicates substance, and provides credibility to the signal. In line with treaty law’s finality to establish and maintain international cooperation, this tempers political confrontation and, by sending information and reducing uncertainty, creates a basis for extra-regime cooperation. The article then demonstrates how and what the TPNW signals, namely that nuclear weapons are illegal, immoral, and dangerous and that nuclear disarmament should advance. The article contends that the TPNW most effectively signals that its States parties want nothing to do with nuclear weapons. Thereby, the TPNW resembles treaties establishing nuclear weapons free zones (NWFZ) and provides a basis for negative security assurances (NSA), which would represent a form of contracting between TPNW States parties and nuclear weapon states. The article concludes that assessments of political effects of international treaties need to consider their formality and legal consequences to a greater extent.