The Sri Lanka Army (Sinhala: ශ්‍රී ලංකා යුද්ධ හමුදාව, romanised: Śrī Laṃkā yuddha hamudāva; Tamil: இலங்கை இராணுவம், romanised: Ilankai iraṇuvam) is the oldest and largest of the Sri Lanka Armed Forces. The army was officially established as the Ceylon Army in 1949, though the army traces its roots back in 1881 when Ceylon Light Infantry Volunteers was created; the army was renamed as the 'Sri Lanka Army' when Sri Lanka became a republic in 1972. In 2024, the Army had approximately 250,000 personnel.

The Army Headquarters is situated in Sri Jayawardenepura Kotte; Commander of the Army is the highest appointment in the army who commands the army and is assisted by the Chief of Staff of the Army and Deputy Chief of Staff of the Army. The Commander-in-Chief of the Sri Lanka Armed Forces is the President of Sri Lanka, who heads the National Security Council through the Ministry of Defence, which is charged with formulating, executing defence policy and procurements for the armed forces.

Repeated incursions by South Indians, particularly the Cholas, into Sri Lankan territory occurred throughout the next few centuries and led to the engagement of the rival forces in battle. In one famous encounter, Sinhalese King Dutugamunu (161–37 BC) raised an army of eleven thousand in his battle against the Chola invader Elara, whom he eventually defeated. Dutugemunu's organisational skills, bravery and chivalry are famous and his battles have gone down in history as outstanding offensive operations.

Other Sri Lankan monarchs whose military achievements stand out include Gajabahu I (113–35), who sailed to India to bring back his captured soldiers, and Dhatusena (455–73) who is credited with repulsing numerous Indian invasions and for organising a naval build-up to deter seaborne attacks. He also had the foresight to cover his defences with artillery. Vijayabahu I (1055–1110) was another warrior king who dislodged Indian invaders and united the country. Parakramabahu the Great (1153–1186) was an outstanding monarch of the Polonnaruwa period, and his accomplishments as a military leader and a great administrator are noteworthy. His reign included a military expedition to Burma in retaliation for indignities inflicted on his envoys and Burmese interference in the elephant trade. This marked the first overseas expedition in Sri Lankan military history. It is also reported that Parakramabahu's fame was such that his assistance was sought by South Indian rulers who were involved in internecine struggles. Another strong ruler in the Transitional period of Sri Lanka was Parakramabahu VI, who defeated Indian invaders, united the island and ruled it from capital Kotte. Although the known epigraphical records do not indicate that the Sri Lankan rulers had a full-time standing army at their disposal, there is evidence supported by legend, designation, name, place and tradition that prove there were 'stand-by' equestrian, elephant, and infantry divisions to ensure royal authority at all times. Militias were raised as the necessity arose, and the soldiers returned to their pursuits, mainly for farming, after their spell of military duty.

Parts of Sri Lanka came under the control of three colonial European powers, namely the Portuguese in the 16th century, the Dutch in the 17th century and the British in the 18th century. Yet, until the entire island was ceded to the British in 1815, regional kingdoms maintained most of their independent defence forces and were able to successfully repulse repeated thrusts by the European armies. However the British, unlike their counterparts, were not primarily restricted to maritime power, and thus had the capability to bring the entire island under their control and to integrate locals into the British defence forces.

At the beginning of the 16th century, modern Europe first came in contact with Sri Lanka. In 1505 a Portuguese fleet, while operating in the Indian seas against Arab traders, was blown off course and landed at Galle, on the southern coast of the island. In 1517 the Portuguese re-appeared, and with the consent of the Sinhalese King established a trading post in Colombo. Having initiated contact with Sri Lanka as traders, the Portuguese soon made themselves political masters of the western seaboard. Numerous forts were soon established, and features of European civilisation was introduced.

The Portuguese are credited with the introduction of European-style fortresses to Sri Lanka during this era. Although some locals already possessed military training and fighting experience, there is no evidence that the Portuguese employed local inhabitants into their own forces. Thus the Portuguese were forced to restrict their presence in the island due to their small numbers and their efforts were more focused toward projecting maritime power.

In 1602 Dutch explorers first landed in Sri Lanka. By 1658 they had completely ousted the Portuguese from the coastal regions of the island. Much like the Portuguese, they did not employ locals in their military and preferred to live in isolation, pursuing their interests in trade and commerce. Like the Portuguese, they defended their forts with their own forces, but unlike the Portuguese, Dutch forces employed Swiss and Malay mercenaries. The Dutch Forts in Jaffna, Galle, Matara, Batticaloa and Trincomalee were sturdily built and are considered a tribute to their military engineering skills. Also, like the Portuguese, the Dutch focussed on maritime power and although they had the capability to develop and use local forces, they chose to isolate themselves from the local population.