Hashomer (Hebrew: השומר‎) ("The Watchman") was a Jewish defense organization in Palestine founded out of Bar-Giora in April 1909. It ceased to operate after the founding of the Haganah in 1920. The purpose of Hashomer was to provide guard services for Jewish settlements in the Yishuv, freeing Jewish communities from dependence upon foreign consulates and Arab watchmen for their security. It was headed by a committee of three - Israel Shochat, Israel Giladi, and Mendel Portugali.

Hashomer was originated by Socialist Zionists, mostly members of Poale Zion, including Israel Shochat, Manya Shochat, Yitzhak Ben-Zvi and Ben-Zvi's wife Rachel Yanait, several of whom had earlier formed a small secret guard society called Bar-Giora, which guarded the Sejera commune (now Ilaniya) and Mes'ha (now Kfar Tavor). Bar-Giora was founded by Israel Shochat, Alexander Zeid and Yitzhak Ben-Zvi, among others. Whilst earlier settlers had undertaken to defend their lands and communities, Hashomer was the first attempt to provide an organized defence for all the Jewish communities in Palestine. A serious obstacle was the lack of funds with which to buy arms. On Yehoshua Hankin's advice, they asked Eliahu Krause, the manager at Sejera, to lend them the money. The first guns were bought, several of the members refusing to part from them even for a moment.