The Nyon Conference was a diplomatic conference held in Nyon, Switzerland, in September 1937 to address attacks on international shipping in the Mediterranean Sea during the Spanish Civil War. The conference was convened in part because Italy had been carrying out unrestricted submarine warfare, although the final conference agreement did not accuse Italy directly; instead, the attacks were referred to as "piracy" by an unidentified body. Italy was not officially at war, nor did any submarine identify itself. The conference was designed to strengthen non-intervention in the Spanish Civil War. The United Kingdom and France led the conference, which was also attended by Bulgaria, Egypt, Greece, Romania, Turkey, the Soviet Union and Yugoslavia.

The first agreement, signed on 14 September 1937, included plans to counterattack aggressive submarines. Naval patrols were established; the United Kingdom and France were to patrol most of the western Mediterranean and parts of the east, and the other signatories were to patrol their own waters. Italy was to be allowed to join the agreement and patrol the Tyrrhenian Sea if it wished. A second agreement followed three days later, applying similar provisions to surface ships. Italy and Germany did not attend, although the former took up naval patrols in November. In marked contrast to the actions of the Non-Intervention Committee and the League of Nations, this conference succeeded in preventing attacks by submarines.

Nyon has been characterised as 'an appeasers paradise. The fiction that attacks on merchant shipping in the Mediterranean was the fault of 'pirates unknown' was fully indulged. [It] preserved the naval status quo in the Mediterranean until the end of the Spanish Civil War: the Francoists received whatever they wanted, the Republicans got very little.'