Year 738 (DCCXXXVIII) was a common year starting on Wednesday of the Julian calendar, the 738th year of the Common Era (CE) and Anno Domini (AD) designations, the 738th year of the 1st millennium, the 38th year of the 8th century, and the 9th year of the 730s decade. The denomination 738 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.

• Charles Martel, Merovingian mayor of the palace, begins a campaign against the Saxons (in modern-day Westphalia) on the northeast frontier. They are subdued and must pay him tribute.^[1]
• Moors under Uqba ibn Al-Hajjaj cross the Pyrenees into France. Uqba fortifies Narbonne and reconquers Avignon, Arles, Nimes. He then advances into Provence, and penetrates as far as Piedmont; he then heads North, and conquers Dauphiné, destroying the city of Saint-Paul, taking Valence, Vienne and Lyon, which he uses as a base to attack Bourgogne.
• Kormesiy, ruler (khagan) of the Bulgarian Empire, is deposed by the nobility. He is replaced on the throne by his son Sevar, who is a descendant of the royal Dulo clan.
• Felice Cornicola is appointed hypatos (Byzantine consul) and magister militum of Venice.

• King Swæfberht of Essex dies after a 23-year reign. He is succeeded by Saelred, a minor member of the Essex royal family.

• The Mayan city-state Xukpi (Copán) is defeated by a rival city-state, Quiriguá. Xukpi leader Uaxaclajuun Ub'aah K'awiil ("Eighteen Rabbit") is deposed thereafter.

• Boniface visits Rome, and is made papal legate of the Frankish Kingdom. He establishes many bishoprics in Bavaria.

• Abu Yusuf, Arab jurist and chief adviser (approximate date)
• Chengguan, Chinese Buddhist monk (d. 839)

• January 26 – John of Dailam, Syrian monk (b. 660)
• May 3 – Uaxaclajuun Ub'aah K'awiil, Mayan ruler (ajaw)
• Áed mac Colggen, king of the Uí Cheinnselaig (Ireland)
• Asad ibn Abdallah al-Qasri, Arab governor
• Dluthach mac Fithcheallach, king of Uí Maine (Ireland)
• Fáelán mac Murchado, king of Leinster
• Maslama ibn Abd al-Malik, Arab general
• Suluk, Turkic ruler (khagan) of the Turgesh
• Swæfberht, king of Essex

• In the fiction of H. P. Lovecraft, Abd Al Azred, Muslim-kafir scholar and scientist, is killed in Damascus city market. His treatise on religion, the Al-Azif, is published soon thereafter.