|Centuries:||6th century – 7th century – 8th century|
|Decades:||640s 650s 660s – 670s – 680s 690s 700s|
|Years:||675 676 677 – 678 – 679 680 681|
|678 by topic|
|State leaders – Sovereign states|
|Birth and death categories|
|Births – Deaths|
|Establishment and disestablishment categories|
|Establishments – Disestablishments|
|Ab urbe condita||1431|
|Chinese calendar||丁丑年 (Fire Ox)
3374 or 3314
— to —
戊寅年 (Earth Tiger)
3375 or 3315
|- Vikram Samvat||734–735|
|- Shaka Samvat||600–601|
|- Kali Yuga||3779–3780|
|Minguo calendar||1234 before ROC
|Seleucid era||989/990 AG|
|Thai solar calendar||1220–1221|
Year 678 (DCLXXVIII) was a common year starting on Friday (link will display the full calendar) of the Julian calendar. The denomination 678 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.
- Autumn – Siege of Constantinople: Emperor Constantine IV confronts the Arab besiegers in a head-on engagement. The Byzantine fleet, equipped with Greek fire, destroys the Muslim fleet at Sillyon., ending the Arab threat to Europe, and forcing Yazid (a son of caliph Muawiyah I) to lift the siege on land and sea.
- King Æthelred of Mercia defeats the Northumbrian forces under king Ecgfrith in a battle near the River Trent. Archbishop Theodore helps to resolve differences between the two, Æthelred agreeing to pay a weregild to avoid any resumption of hostilities (approximate date).
- April 27 – Emperor Tenmu holds divination for the purpose of proceeding to the Abstinence-palace.
- May 3 – Princess Tōchi suddenly takes ill and dies within the palace. Tenmu, her father, is unable to sacrifice to the Gods of Heaven and Earth.
- May 10 – Tōchi is buried at a place which could be Akō (Hyōgo Prefecture). Tenmu is graciously pleased to raise lament for her.
- Wilfrid, bishop of York, is at the height of his power and owns vast estates throughout Northumbria. After his refusal to agree to a division of his see, Ecgfrith and Theodore, archbishop of Canterbury, have him banished from Northumbria.
- April 11 – Pope Donus dies at Rome after a reign of 1 year and 160 days. He is succeeded by Agatho I who becomes the 79th pope. He is the first pope to stop paying tribute to Emperor Constantine IV upon election.
- In Japan, the national worshiping to the Gods of Heaven and Earth is planned. Tenmu tries to select his daughter Tōchi as a Saiō to make her serve the Gods. However, Tōchi suddenly takes ill and dies.
- The Beomeosa-templecomplex in Geumjeong-gu (modern South Korea) is constructed during the reign of king Munmu of Silla.
- Abdullah ibn Aamir, Arab general (b. 622)
- Ælfwine, king of Deira (approximate date)
- Aisha, wife of Muhammad
- Arbogast, bishop of Strasbourg
- April 11 – Pope Donus
- Nathalan, Scottish bishop
- May 3 – Tōchi, Japanese princess
- Wechtar, Lombard duke of Friuli
- Zhang Wenguan, chancellor of the Tang dynasty (b. 609)
- Haldon (1990), p. 64; Lilie (1976), pp. 78-79; Treadgold (1997), pp. 326-327; Mango & Scott (1997), p. 494