|Centuries:||6th century – 7th century – 8th century|
|Decades:||600s 610s 620s – 630s – 640s 650s 660s|
|Years:||635 636 637 – 638 – 639 640 641|
|638 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1391|
|Chinese calendar||丁酉年 (Fire Rooster)
3334 or 3274
— to —
戊戌年 (Earth Dog)
3335 or 3275
|- Vikram Samvat||694–695|
|- Shaka Samvat||560–561|
|- Kali Yuga||3739–3740|
|Minguo calendar||1274 before ROC
|Seleucid era||949/950 AG|
|Thai solar calendar||1180–1181|
Year 638 (DCXXXVIII) was a common year starting on Thursday (link will display the full calendar) of the Julian calendar. The denomination 638 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.
- Emperor Heraclius creates a buffer zone or no man's land in the heartland of Asia Minor. In the mountainous terrain of Anatolia, the Byzantine forces develop a system of defensive guerrilla warfare. The strategy is known as ‘shadowing warfare’, as it avoid battle with major Muslim invasions and instead attacking raiding parties on their return when they are laden with booty, captured livestock or prisoners.
- July 4 – Heraklonas, age 12, son of Heraclius obtains through the influence of his mother Martina the title of Augustus. This brings him in rivalry with his elder half-brother Constantine.
- Heraclius issues his Ekthesis espousing the Monothelete doctrine (that there is only one will in Christ) and setting it forth as the official doctrine of the Eastern Orthodox Church. The Ekthesis is vigorously opposed, notably by Maximus the Confessor.
- King Oswald and his Northumbrian army besiege and conquer Edinburgh (Scotland). His half-brother, Oswiu of Bernicia, marries princess Rhiainfelt, heiress of North Rheged ("Old North"). Northumbria embraces North Rheged in a peaceful takeover and Oswiu becomes a sub-king (approximate date).
- Arab-Byzantine War: The invading Rashidun army under Khalid ibn al-Walid move into Anatolia conquering without strong Byzantine resistance, the cities Kahramanmaraş, Caesarea Cappadociae, Sebastia, and Malatya (west of the Taurus Mountains). Arab forces march into Armenia where they capture the cities Edessa and Amida up to the Ararat plain.
- Autumn – The Islamic forces under Abu Ubaidah ibn al-Jarrah storming Caesarea Maritima, capital of Byzantine Palestine, and effecting their final capture of Ascalon (modern Israel). Caliph Umar I stops the Muslim invasion and appoints Abu Ubaidah to governor of Syria.
- Umar I dismisses Khalid ibn al-Walid after the conquest of Syria, owing to his ever-growing fame and influence. He wants the Muslims to know that victory come from God, not his general. The Persian Empire including Bactria, Caucasus, and Makran are annexed to the Rashidun Caliphate.
- Abu Musa al-Asha'ari, companion (sahabah) of Muhammad, establishes Hafar Al-Batin located in the northeastern region of the Arabian Peninsula. He orders the digging for new wells along this desert route that Muslims travel from Iraq to Mecca for the Hajj (pilgrimage).
- The Tibetan Empire, seeking a marriage alliance with Tang dynasty China, launches an attack on Songzhou that is repelled by Chinese forces, but is followed by the marriage of the Chinese Princess Wencheng to Tibetan ruler Songtsän Gampo.
Arts and sciences
- The Islamic calendar is introduced by Abu Musa al-Asha'ari. He convinces Umar I to make notes of an era for Muslims.
- March 22 – Year 0 of the Burmese calendar, based on the Chula Sakarat also used in the mainland of Southeast Asia.
- October 12 – Pope Honorius I dies at Rome after a 13-year reign and is succeeded by Severinus, but the Byzantine emperor Heraclius will delay the new pope's consecration until May 640.
- December 20 – Pyrrhus I becomes patriarch of Constantinople after the death of Sergius I. He has been an advocate of Monothelitism and a close friend of Heraclius.
- March 11 – Sophronius, patriarch of Jerusalem
- October 12 – Pope Honorius I
- December 9 – Sergius I, patriarch of Constantinople
- Qin Shubao, general of the Tang Dynasty
- Yu Shinan, calligrapher and official (b. 558)
- The Great Islamic Conquests AD 632–750. David Nicolle (2009), p. 52. ISBN 978-1-84603-273-8