Roman emperor Trajan inaugurates the Aqua Traiana, an aqueduct that channels water from Lake Bracciano, 40 kilometres (25 miles) northwest of Rome.

Year 109 (CIX) was a common year starting on Monday of the Julian calendar. At the time, it was known as the Year of the Consulship of Palma and Tullus. The denomination 109 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.

Trajan was a Roman emperor from AD 98 to 117, remembered as the second of the Five Good Emperors of the Nerva–Antonine dynasty. He was a philanthropic ruler and a successful soldier-emperor who presided over one of the greatest military expansions in Roman history, during which, by the time of his death, the Roman Empire reached its maximum territorial extent. He was given the title of Optimus by the Roman Senate.

The Aqua Traiana was a 1st-century Roman aqueduct built by Emperor Trajan and inaugurated in 109 AD. It channelled water from sources around Lake Bracciano, 40 km (25 mi) north-west of Rome, to ancient Rome. It joined the earlier Aqua Alsietina to share a common lower route into Rome.

The Romans constructed aqueducts throughout their Republic and later Empire, to bring water from outside sources into cities and towns. Aqueduct water supplied public baths, latrines, fountains, and private households; it also supported mining operations, milling, farms, and gardens.

Lake Bracciano is a lake of volcanic origin in the Italian region of Lazio, 32 km (20 mi) northwest of Rome. It is the second largest lake in the region and one of the major lakes of Italy. It has a circular perimeter of approximately 32 km (20 mi). Its inflow is from precipitation runoff and percolation, and from underground springs, and its outflow is the Arrone.