Field Marshal William Gustavus Nicholson, 1st Baron Nicholson, GCB (2 March 1845 – 13 September 1918) was a British Army officer who served in the Second Anglo-Afghan War, the Mahdist War, the Third Anglo-Burmese War, the Second Boer War and the First World War. He became Chief of the Imperial General Staff and was closely involved in the reorganisation of the British Army in the early years of the 20th century.

Nicholson was born in Roundhay, Leeds, the second-youngest son of William Nicholson Nicholson (who had been born with the surname of Phillips but in 1827 assumed his mother's surname of Nicholson) and Martha Nicholson (née Rhodes). Nicholson graduated from Leeds Grammar School in 1863 and entered the Royal Military Academy, Woolwich, where he was awarded the Pollock Medal the following year.

Nicholson was commissioned as a lieutenant in the Royal Engineers on 21 March 1865. From 1868 to 1871 he was employed on coastal fortification work in Barbados, West Indies. Following this he was posted to India, with the Public Works Department at Hyderabad, the Punjab Irrigation Branch, and at Rawalpindi and Peshawar on barrack work and the construction of Army waterworks.

Nicholson was promoted to the rank of captain on 16 March 1878, and posted to Afghanistan, seeing service in the Second Anglo-Afghan War. He served in the first campaign as Field Engineer, Kandahar Field Force from 10 October 1878 to 5 March 1879, and as Royal Engineers Commander for the Thal-Chotiali Field Force from 6 March to 30 April 1879. During the second campaign of the war he served first as Field Engineer, 1st Division, Kabul Field Force from 23 September 1879 until 7 August 1880, being present at the action near Surkai Kotal on 14 October 1879, the defence of the Shutargardan in October 1879 and the defence of the Lataband in December 1879. He then served as Field Engineer, Kabul-Kandahar Field Force, taking part in the advance to the relief of Kandahar, and being present at the Battle of Kandahar. During the Afghanistan campaigns, he was three times mentioned in despatches and was awarded a campaign medal with three clasps.

He was appointed Secretary of the Defence Committee at Simla in 1880 and was given the brevet rank of major on 1 March 1881. His time as secretary was interrupted by service in Egypt in 1882, where he served with the Indian contingent in the Egyptian Campaign. His force made a successful flanking movement at the battle of Tel-el-Kebir and opened up the way to Cairo by cutting the enemy's railway system near Zagazig, where Nicholson, then with the cavalry, captured four trains under steam, which were later used to transport British infantry. His efforts earned him another campaign medal with clasp, the Order of Osmanieh (4th Class), and the Khedive's Star. He was promoted to the substantive rank of major on 21 March 1885.

From 1885 he served as Assistant Adjutant General, Royal Engineers in Bengal. Service in the Third Anglo-Burmese War, which stamped out the guerrilla activities that followed the overthrow of King Thibaw Min, earned Nicholson a further mention in despatches and promotion to the rank of brevet lieutenant colonel on 1 July 1887.

Nicholson was appointed Military Secretary to Lord Roberts, Commander-in-Chief in India on 1 July 1890 and granted the substantive rank of colonel on 1 January 1891. He was appointed a Companion of the Order of the Bath in the Queen's Birthday Honours 1891. He was employed with the Military Works Department, India as a chief engineer from 1893 and was appointed Adjutant General for the Punjab with rank of brigadier general in 1895.

Nicholson saw service on the North West Frontier of India as Chief of Staff for the Tirah Campaign in 1897 to 1898. Lieutenant General Sir William Lockhart mentioned him in despatches referring to his "brilliant abilities" on 29 March 1898. He was awarded a campaign medal with two clasps and advanced to a Knight Commander of the Order of the Bath on 20 May 1898. He was made Adjutant-General in India on 24 February 1899.