On 25 June 1950, the (North) Korean People's Army invaded the Republic of (South) Korea. The invasion was seen as a challenge to the western democracies, a consequence of the growing Cold War tensions between the East and West. The United Nations Security Council invited UN member states to send forces to restore the situation; under the auspices of United Nations Command - Korea (UNC-K).

In late September 3RAR joined the 27^th British Commonwealth Brigade. 3RAR took part in the long advance into North Korea and in October was in action less than 100 kilometres from the Yalu River, which formed the Chinese border. With the entry of Chinese forces into the conflict in November, a long withdrawal was undertaken back behind the 38^th Parallel.¹³⁷

In April 1951 another Chinese offensive saw 3RAR and 2^nd Battalion Princess Particia's Canadian Light Infantry (2PPCLI) committed to a hard-fought defensive battle at Kapyong to stop a Chinese division. For this action these battalions and one other unit were awarded the US Presidential Unit Citation.¹³⁸ Succumbing to US pressure, Australia agreed in October to send a second Battalion to Korea. Several other members of the Commonwealth increased their contribution, to the extent that a British Commonwealth Division could be formed, with an Australian commanding one of the Brigades.¹³⁹

A UN offensive in October 1951 saw the newly formed 1^st Commonwealth Division involved in Operation Commando when UN forces extended their positions along the Jamestown Line. During this operation, 3RAR took the strongly defended Chinese positions on Maryang San.¹⁴⁰ In November 1951 the Korean War entered its static phase which lasted until a ceasefire was agreed on 27 July 1953.

In April 1952 3RAR was joined by 1RAR , the brigade having been redesignated a year previously as 28 Commonwealth Brigade. Command of this brigade, with two RAR battalions, was from July 1952 vested in Australian brigadiers. 1RAR was relieved by 2RAR in March 1953 with 3RAR, with its system of individual rather than unit replacement, remained committed until the end.¹⁴¹

The static phase of the war saw the RAR battalions involved in extensive patrolling and trench raids, as well as the major actions of Hill 355.and the Hook. 1RAR relieved 2RAR in April 1954 and 3RAR returned to Australia in November 1954 leaving 1RAR as a component of a reduced Commonwealth commitment to the UN force in Korea.¹⁴²

Australian Army casualties totalled 276 killed in action or died of wounds, 1210 wounded in action and 23 prisoners of war.¹⁴³