The Akāl Sena (Gurmukhi: ਅਕਾਲ ਸੈਨਾ; meaning 'Army of the Immortal', 'God's Army', or 'Eternal Army'; alternatively transcribed as Akaal Sena) was the Sikh military force established by the sixth Sikh Guru, Guru Hargobind. It was the first standing Sikh army. It was also known as the Akali Dal (Gurmukhi: ਅਕਾਲੀ ਦਲ, 'Immortal Brigade').

During the time period of Guru Arjan, an enemy of the Sikhs and the brother of Arjan, named Prithi Chand, instigated a local Mughal official named Sulahi Khan to destroy the Sikhs and the Guru. Sulahi Khan conjured up an excuse that he was collecting tax to justify him leading a small contingent against the Sikhs at Amritsar. Due to the local residents of Amritsar fearing for their personal safety, Guru Arjan left the city to prevent tragedy. Arjan made his way to Wadali and then from there to Raur. After Wadali was ransacked by bandits, Guru Arjan returned and stayed there for two years to provide security for the local residents.

Guru Arjan had advised his son, Hargobind, to become martially trained when he was a youth. The Akal Sena came into being at the same time of the consecration of the Akal Bunga on the 15 June 1606. Guru Hargobind believed that the Sikh faith was not limited to spiritual pursuits but martial power and temporal authority was vital as well. There are various views as to why the Sikh community and faith was martialized, with one theory being it was caused by the growing religious fundamentalism of the Mughal emperors. Some scholars, notably Trilochan Singh, argue that the roots for the martial tradition in Sikhism date back further to the guruship of Guru Arjan, pre-dating Guru Hargobind's militarization reforms and enactments.

The Akaal Takhat and the Akaal Sena (God's army) the Sikhs, would protect and uphold the integrity of faith promulgated by Harmander Sahib and all it stood for. The Sikhs would become renowned for their bravery as warriors, who protect the integrity and faith of one and all. They would fight oppression and tyranny, relinquishing risk to life; they would make daring attacks on foes, defending their strong principles of righteousness.

— Harjinder Singh, Reflections on 1984 (2014), pages 16–17

It had been described as an "elite army corps" for the Sikh community to meet the "Mughal challenge". Sikhs were called upon to bring horses and equipment for the army, which they obliged. The warriors of the Akal Sena came to be known as Akalis (the immortals).

The army was founded by Guru Hargobind. During his time, it consisted of 700-800 horses, 300-500 cavalrymen, 60 musketeers, and 60 artillery men. The Akal Sena fought major battles against the Mughal empire and its allies, winning all four of the major battles in-which Hargobind was leader, as well as more minor skirmishes. The first commanders of the Akal Sena were Bidhi Chand Chhina, Baba Jattu, Baba Peda, and Baba Praga, commanding a force around 400 strong. Later a special regiment of Pathans led by Painde Khan joined the Akal Sena.

Initially, the Akal Sena was a policing force that was an army only nominally. It was only after the release of the sixth Guru from the prison of Gwalior Fort in 1612 that the force began to take on more shape, as 52 Rajput kings also gained freedom from the prison, who then accompanied the Guru as many of them had lost their polity to conquests and absorptions by the Mughal empire. The Guru established an Akhara (Indic military training centre or arena) the same year in Amritsar, it was called the Ranjit Akhara (alt. spelt as 'Ranjeet'). Rajput converts to the Sikh faith had given the new institution much strength and information regarding the art of war.