Year 688 (DCLXXXVIII) was a leap year starting on Wednesday (link will display the full calendar) of the Julian calendar. The denomination 688 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.
|Ab urbe condita||1441|
|Balinese saka calendar||609–610|
|Chinese calendar||丁亥年 (Fire Pig)|
3384 or 3324
— to —
戊子年 (Earth Rat)
3385 or 3325
|- Vikram Samvat||744–745|
|- Shaka Samvat||609–610|
|- Kali Yuga||3788–3789|
|Minguo calendar||1224 before ROC|
|Seleucid era||999/1000 AG|
|Thai solar calendar||1230–1231|
814 or 433 or −339
— to —
815 or 434 or −338
- Byzantine–Bulgarian War: Emperor Justinian II carries out a Balkan campaign and marches through Thrace, where he restores Byzantine rule. He establishes a theme administration, and migrates many Bulgars and Slavs to the Opsician Theme (Asia Minor).
- Justinian II reestablishes Byzantine settlement on Cyprus, signing a treaty (and paying an annual tribute) with Umayyad caliph Abd al-Malik, for joint occupation of the island.
- King Perctarit of the Lombards is assassinated by a conspiracy, after a 17-year reign. He is succeeded by his son Cunipert, who is crowned ruler of the Lombard Kingdom in Italy.
- Alahis, duke of Brescia, starts a civil war in Northern Italy. He besieges Cunipert on an island in Lake Como (Lombardy), who breaks out with Piedmontese troops.
- King Caedwalla of Wessex abdicates the throne and departs on a pilgrimage to Rome, possibly because of the wounds he suffered while fighting on the Isle of Wight. The power vacuum is filled by Ine, son of his second cousin, sub-king Coenred of Dorset.
- King Æthelred of Mercia establishes Mercian dominance over most of Southern England. He installs Oswine, minor member of the Kentish royal family (second cousin of king Eadric), as king of Kent. Prince Swæfheard of Essex is given West Kent.
- Fine, John V. A. Jr. (1991) . The Early Medieval Balkans: A Critical Survey from the Sixth to the Late Twelfth Century. Ann Arbor, Michigan: University of Michigan Press. p. 71. ISBN 0-472-08149-7.
- Yorke, Barbara (1990), "Kings and Kingdoms of Early Anglo-Saxon England", London: Seaby, ISBN 1-85264-027-8