Nearchus or Nearchos (Greek: Νέαρχος; c. 360 – 300 BC) was one of the Greek officers, a navarch, in the army of Alexander the Great. He is known for his celebrated expeditionary voyage starting from the Indus River, through the Persian Gulf and ending at the mouth of the Tigris River following the Indian campaign of Alexander the Great, in 326–324 BC.

A native of Lato in Crete and son of Androtimus, his family settled at Amphipolis in Macedonia at some point during Philip II's reign (we must assume after Philip took the city in 357 BC), at which point Nearchus was probably a young boy. He was almost certainly older than Alexander, as were Ptolemy, Erigyius, and the others of the ‘boyhood friends’; so depending on when Androtimus came to Macedonia Nearchus was quite possibly born in Crete. Nearchus, along with Ptolemy, Erigyius and Laomedon, and Harpalus, was one of Alexander's ‘mentors’ – and he was exiled by Philip as a result of the Pixodarus affair (A 3.6.5; P 10.3). It is not known where the exiles went, but they were recalled only after Philip's death, on Alexander's accession.

After their recall, these men were held in the highest honour. Nearchus was appointed as satrap of Lycia and Pamphylia in 334/3 BC (A 3.3.6), one of the earliest of Alexander's satrapal appointments. Nearchus' naval blockade of Persian fleets threatening the Aegean Sea was successful in aiding Alexander's conquest of Phoenicia, Egypt and Babylonia. In 328 BC he was relieved of his post and rejoined Alexander in Bactria (northern Afghanistan), bringing with him reinforcements (A 4.7.2; C 7.10.4, but does not mention Nearchus himself). After the siege of Aornos in present-day Khyber Pakhtunkhwa, Pakistan, Nearchus was sent at the head of a reconnaissance mission – especially to find out about elephants (A 4.30.5–6).

In 326 BC, Nearchus was made admiral of the fleet that Alexander had constructed at the Hydaspes (A 6.2.3; Indica 18.10). However, his trierarchy was a financial responsibility – that is, Nearchus put up the money for the boats (Heckel, p.229); and there were plenty of other trierarchs in the Indus fleet who were not natural-born sailors. Strabo recounts that the Himalayan range of Emodus was close to the construction of the fleet near Taxila, providing ample supplies of fir, pine and cedar timber. Initially, the fleet progressed down the Hydaspes much like a triumphal military parade, accompanied by a land-based entourage of the main armed forces of Alexander including cavalry, elephants and loot trains. At the confluence of the Acesines and Indus Rivers, Alexander founded a city called Alexandria-on-the-Indus, assigning it to the satrapy of Oxyartes (father of Roxana) and populating it with Thracian troops.

Some of the ships were damaged, and Nearchus was instructed to remain behind to oversee repairs, before continuing down the river. This perhaps indicates some knowledge of shipbuilding, but he could hardly have been the only one qualified. The voyage down the Indus River lasted from 326 to 325 BCE and resulted in the capture of native Indian towns. By the time the Macedonians had reached Pattala (modern Bahmanabad in Sindh, Pakistan), Nearchus prepared to lead 17–20,000 men for an expedition into the Persian Gulf, while Alexander continue through the Gedrosian desert. Nearchus was not the only Greek naval officer to have pursued a voyage down the Indus River—this was also done by Scylax of Caryanda under the commission of Darius the Great, according to Herodotus.

Nearchus remained in command of the fleet for the voyage from the Indus to the Persian Gulf, which he recorded in detail (and which was used extensively for Arrian’s Indica). Again, although he was the admiral, in command of the fleet, great seamanship was not required – the naval responsibilities were Onesicritus’. Nearchus compiled the story of his expedition into a written work—the Indike—which is now lost but informs some of the content in Arrian's Indica and Strabo's Geographica. This work likely consisted of two parts: one detailing India's frontiers, size, population, castes, fauna, flora, cultures and militaries, and the other describing his home-bound voyage toward Babylon. Nearchus described, according to Arrian, how commodities like rice, sugarcane and cotton fabrics and textiles were cultivated, manufactured and traded in the Indus Valley.

Nearchus began by setting out from Patala, although monsoon rains and heavy winds delayed his reaching the Arabian Sea. To wait out the adverse weather, the Macedonian fleet camped near the mouth of the Indus and Arabius Rivers, building stone walls as fortification against hostile natives and subsisting off of briny water, mussels, oysters and razor-fish. After 24 days, Nearchus continued on to the harbor of Morontobara (Manora Island just off the coast of modern Karachi, Pakistan). About Morontobara, Arrian writes:

At Morontobara, Leonnatus (one of Alexander's Generals) had defeated the local Oreitans and deposited a food supply from which Nearchus' fleet subsisted. Resupplied, Nearchus reached the Hingol River (in Makran, Balochistan) and destroyed the native population. Nearchus had arrived at the country of the Icthyophagoi -- 'Fish-Eaters' -- who inhabited the barren coastal region of Makran, between the Arabian Sea and the Gedrosian Desert and found the harbor of Bagisara (modern Ormara Port).