Guru Arjan (Gurmukhi: ਗੁਰੂ ਅਰਜਨ, pronunciation: [gʊɾuː əɾd͡ʒənᵊ]; 15 April 1563 – 30 May 1606) was the fifth of the ten Sikh Gurus. He compiled the first official edition of the Sikh scripture called the Adi Granth, which later expanded into the Guru Granth Sahib. He is regarded as the first of the two Gurus martyred in the Sikh faith. He is credited for founding the settlements of Kartarpur (Jalandhar district), Hargobindpur, and Tarn Taran.

Guru Arjan was born in Goindval, in the Punjab, the youngest son of Bhai Jetha, who later became Guru Ram Das, and Mata Bhani, the daughter of Guru Amar Das. He completed the construction of the Darbar Sahib at Amritsar, after the fourth Sikh Guru, his father, had founded the town and built a sarovar. Arjan compiled the hymns of previous Gurus and of other saints into the Adi Granth, the first edition of the Sikh scripture, and installed it in the Harimandir Sahib.

Guru Arjan reorganized the Masand system initiated by Guru Ram Das by suggesting that Sikhs donate, if possible, one-tenth of their income, goods or service to the Sikh organization (a practice known as dasvandh). The Masand was an individual who not only collected these funds but also taught tenets of Sikhism and settled civil disputes in their region, similar to a vicar or parish priest in Christianity. The dasvandh financed the building of gurdwaras and langars (shared communal kitchens).

Guru Arjan was arrested under the orders of the Mughal Emperor Jahangir, who accused him of supporting a rebellion under Khusrau Mirza. The emperor demanded that he convert to Islam, but he refused, so he was tortured and executed in 1606. Historical records and the Sikh tradition are unclear as to whether Arjan was executed by drowning or died during torture. The Sikh tradition states the Guru's execution was a part of the ongoing persecution of the Sikhs under the Mughal Empire. His martyrdom is considered a watershed event in the history of Sikhism. It is remembered as Shaheedi Divas of Guru Arjan in May or June according to the Nanakshahi calendar released by the Shiromani Gurdwara Parbandhak Committee in 2003.

Guru Arjan was born in Goindval to Bibi Bhani and Jetha Sodhi. Bibi Bhani was the daughter of Guru Amar Das, and her husband Jetha Sodhi later came to be known as Guru Ram Das. Arjan's birthplace site is now memorialized as the Gurdwara Chaubara Sahib. He had two brothers: Prithi Chand and Mahadev. Various Sikh chroniclers give his birth year as 1553 or 1563, the latter is accepted by scholarly consensus as the actual year of birth with 15 April as the accepted birth date.

Guru Arjan spent the first 11 years of his life in Goindwal and the next seven years with his father in Ramdaspur. Per Sikh tradition, he had stayed for two years in Lahore during his youth after being sent by his father to attend the wedding of his first cousin Sahari Mal's son as well as to establish a Sikh congregation. He was appointed as the Sikh Guru in 1581 after the death of his father. Guru Ram Das was a Khatri of the Sodhi sub-caste. With Arjan's succession, the Guruship remained in the Sodhi family of Guru Ram Das.

According to historians, Guru Arjan wedded twice. His first wife was Mata Ram Dei, whom he married on 20 June 1579. His second wife was Mata Ganga, their wedding taking place on 19 May 1589. Popular Sikh tradition omits his first wife and claims he was only married to Ganga. According to Mohinder Kaur Gill, Sikh tradition states that Mata Ram Dei could not produce a child for Guru Arjan, thus she persuaded him to marry another woman. Another version claims Mata Ram Dei died so the guru married again.

Guru Ram Das chose Arjan, the youngest, to succeed him as the fifth Sikh Guru. Mahadev, the middle brother chose the life of an ascetic. His choice of Guru Arjan as successor, as throughout most of the history of Sikh Guru successions, led to disputes and internal divisions among the Sikhs.