Year 775 (DCCLXXV) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. The denomination 775 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.

• September 14 – Emperor Constantine V dies while on a campaign in Bulgaria. In his 34-year reign he has suppressed monasticism and image worship, restored aqueducts, revived commerce, and repopulated Constantinople^{[citation needed]}. He is succeeded by his 25-year-old son Leo IV ("the Khazar"), who continues Constantine's campaigns against the Bulgars and Muslim Arabs.

• Saxon Wars: King Charlemagne holds a major assembly at Quierzy (Northern France). He leads a Frankish army into Saxony to retake the castrum of Syburg (near Dortmund), then rebuilds and garrisons fortified Eresburg. He reaches the Weser at a place called Braunsberg, where the Saxons stand for battle, but are defeated when Frankish troops cross the river.^[1]
• Westphalian Saxons, probably commanded by Widukind, cross the Weser and fight an inconclusive battle at Hlidbeck (modern-day Lübbecke). Charlemagne claims victory, but perhaps in reality suffers a setback. He reunites his forces and inflicts a real defeat upon the Saxons, seizing considerable booty and taking hostages, though Widukind escapes.^[2]
• Autumn – Charlemagne retakes the Hellweg (main corridor) along the Lippe Valley, establishing communications between Austrasia, Hesse and Thuringia. It is used as a trade route under Frankish supervision.^[3]
• The German city of Giessen (Hesse) is founded.

• Andalusian merchants set up an emporium (trade settlement) on the Maghreb coast at Ténès (modern Algeria). It is early evidence of the revival of the maritime trade in the Western Mediterranean, after the chaos of the early 8th century.^[4]

• April 25 – Battle of Bagrevand: The Abbasids put an end to an Armenian rebellion. Muslim control over Transcaucasia is solidified, while several major Armenian nakharar families, notably the Mamikonian, lose power and flee to the Byzantine Empire.
• Caliph al-Mansur (r. 754–775) dies after a 21-year reign, in which he has made Baghdad the residence of the Abbasid Caliphate. He is succeeded by his son al-Mahdi.

• At around this time, Baghdad becomes the largest city in the world, taking the lead from Chang'an, capital of China.^[5]

• Tibet subdues her Himalayan neighbors, and concludes a boundary agreement with the Chinese Tang dynasty (approximate date).
• King Dharmapala begins his reign of Bengal (South Asia).^[6]

• A 1.2% growth of carbon-14 concentration recorded in tree rings suggests that a very strong solar storm may have hit Earth, in either 774 or 775.^[7]

• Amalarius, archbishop of Trier (approximate date)
• Ebbo, archbishop of Reims (d. 851)
• Einhard, Frankish scholar (d. 840)
• Fujiwara no Fuyutsugu, Japanese general (d. 826)
• Hilduin, bishop of Paris (d. 840)^{[citation needed]}
• Leo V, Byzantine emperor (d. 820)
• Rotrude, Frankish princess, daughter of Charlemagne (or 778)
• Tahir ibn Husayn, 9th-century Persian Abbasid governor (or 776)
• Theodosia, Byzantine empress (approximate date)
• Theophanes the Branded, Byzantine monk (d. 845)
• Wetti of Reichenau, German scholar (approximate date)

• April 25
  • Smbat VII Bagratuni, Armenian noble
  • Mushegh VI Mamikonian, Armenian noble
• September 14 – Constantine V, Byzantine emperor (b. 718)
• October 6 – Al-Mansur, Muslim caliph (b. 714)
• date unknown
  • Ciniod I, king of the Picts
  • Fujiwara no Kurajimaro, Japanese politician (b. 734)
  • Princess Inoe of Japan (b. 717)^[8]
  • Isma'il ibn Jafar, Shī‘ah Imām (approximate date)
  • Kibi no Makibi, Japanese scholar (b. 695)
  • Ruyuan, Chinese Buddhist abbess and master