Hashomer (Hebrew: השומר, 'The Watchman') was a Jewish defense organization in Palestine founded in April 1909. It was an outgrowth of the Bar-Giora group and was disbanded after the founding of the Haganah in 1920. Hashomer was responsible for guarding 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 on September 29, 1907, by Israel Shochat, Alexander Zaïd, Yehezkel Henkin in the apartment of Yitzhak Ben-Zvi which was in Jaffa. Less than two years later, on April 12, 1909, the Bar-Giora leadership decided at a meeting in Kfar Tavor to disband their organization and create a larger one, Hashomer. While 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. By 1910 Yehezkel Henkin was the first of the Shomer people to ride horseback so he became a riding guard. He taught other “Watchmen” how to ride. This prompted the committee to buy him a horse that he named "Tzipora".

In the autumn of 1911 Manya Shochat wrote, on behalf of Hashomer, to Menachem Ussishkin in Odessa asking for money. In her letter she stated that in 1910 Hashomer had “35 watchmen, 23 infantry and 12 cavalry”, and were guarding six colonies in Galilee as well as Hadera.

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.

They adopted local dress, and many of the customs of the Bedouins, Druze and Circassians. They also drew inspiration from the history of the Cossacks. The first few shomrim (guards), worked on foot, but soon acquired horses, which vastly increased their effectiveness. Mendel Portugali laid down the rules of engagement.

You do not seek an encounter with the thief; you chase him off, and only when you have no choice do you shoot. After all, he is out to steal a bag of grain, not to murder you, so don't murder him, drive him off. Don't sleep at night. If you hear footsteps, fire into the distance. If you feel he is a few steps away and you can fire without him falling upon you, fire into the distance. Only if your life is in danger—fire.

Guns used were the same as the locals', which included the 'jift', a single, or double-barrelled shotgun, the 'yunani' and 'osmanli' single-shot muskets, various rifles and Mauser pistols. Modern rifles, known as 'Abu-Hamsa' (father of five), were the most prestigious, and were prone to theft by the locals. The 'shibriyeh' (Arab dagger) and 'nabut' (club or mace) were carried by all. Ammunition was expensive and hard to come by, so primitive production centers were set up.

By 1912, Hashomer was guarding fourteen Jewish settlements. In addition to guarding settlements, Hashomer secretly began developing offensive capabilities, seeing itself as the nucleus of a future Jewish army. It raided Arab villages to beat or kill residents who had harmed Jews. Mounted on their horses, Hashomer vigilantes raided a few Arab settlements to punish residents who had harmed Jews, sometimes beating them up, sometimes executing them. In one case, a special clandestine assembly of Hashomer members decided to eliminate a Bedouin policeman, Aref al-Arsan, who had assisted the Turks and tortured Jewish prisoners. He was shot dead by Hashomer in June 1916.