The 9th Brigade is a Reserve formation of the Australian Army headquartered at Keswick Barracks in Keswick, South Australia, with elements located in New South Wales and South Australia. The brigade was first raised in 1912 in New South Wales following the introduction of the compulsory training scheme.

During World War I, the brigade was formed as part of the First Australian Imperial Force in 1916, with the majority of its recruits coming from New South Wales. It was assigned to the 3rd Division, and training was carried out in the United Kingdom before the brigade was committed to the fighting on the Western Front in November 1916. It fought in numerous battles in France and Belgium for the next two years.

After the war, the brigade was disbanded, but was re-raised as a part-time formation in the Sydney area. During World War II, the brigade was mobilised for defensive duties, but did not see active service before it was disbanded in June 1944. In the post war period, the 9th Brigade was re-raised once again as a part-time formation, and forms part of the 2nd Division.

The 9th Brigade traces its origins to 1912, when it was formed as a Militia brigade as part of the introduction of the compulsory training scheme, assigned to the 2nd Military District. The brigade's constituent units were spread across various locations in New South Wales including Camperdown, Darlington, Enmore, Newtown, Marrickville, St Peters and Petersham.

During World War I, the 9th Brigade was originally formed as part of the First Australian Imperial Force. Raised from volunteers drawn from New South Wales, the 9th Brigade was formed in 1916 in the period as the AIF was expanded following the failed Gallipoli Campaign. The brigade consisted of four infantry battalions—the 33rd, 34th, 35th and 36th Battalions—as well as the 9th Light Trench Mortar Battery and the 9th Machine Gun Company (which later formed part of the 3rd Machine Gun Battalion). Its first commander was Brigadier General Alexander Jobson.

The brigade was attached to the 3rd Division. After rudimentary training in Australia, the brigade was shipped to England to complete its training before being committed to the fighting on the Western Front in France and Belgium in November 1916. The brigade's first major battle came in mid-1917 when it took part in the Battle of Messines. Later in 1917, it fought during the Battle of Passchendaele before taking part in defensive operations during the German spring offensive in early 1918.

In April 1918 it was involved in the Allied counterattack at the First Battle of Villers-Bretonneux. This was followed by the Second Battle of Morlancourt. The brigade took part in the final offensive of the war, the Hundred Days Offensive. During this time, the 9th Brigade's casualties had been so high, that one of its battalions—the 36th—had to be disbanded in order to reinforce the others. After Jobson, the brigade's other wartime commanders were Charles Rosenthal and Henry Goddard.

Following the end of hostilities the brigade was disbanded in 1919. However, while the AIF had been deployed, a separate Citizens Force (later known as the Militia) formation remained in Australia. By 1918, a 9th Brigade had been established within the 2nd Military District, consisting of the 33rd, 34th, 35th and 36th Infantry Battalions.