Castra Bonna, Roman military fort in Bonn, Germany.

Castra Bonna was a legionary fortress on the Rhine in what is now Bonn, measuring 520 by 516 metres (26.83 hectares) and housing the Legio I Germanica from the campaigns of Germanicus (14–16 CE). Its Via principalis corresponds almost exactly to today's Römerstraße, and its centre lay at the crossing of the Roman road with Nordstraße/Badener Straße. The site lies between today's Augustusring, Rosental, Graurheindorfer Straße and the riverbank.

The fort was founded in the 1st century CE to secure the northern border of the Roman Empire against Germanic tribes. It remained occupied for over 300 years before declining in importance as Roman military presence shifted throughout the region.

The name Bonna survives in today's Bonn, and the fort's Via principalis line matches the modern Römerstraße almost exactly. Two civilian settlements grew beside the camp: the Canabae Legionis, immediately south at today's Wachsbleiche, and the Vicus Bonnensis, further south in the former government quarter. Since the 1820s the LVR-Landesmuseum Bonn and the Verein von Altertumsfreunden im Rheinlande have researched the site.

Most of the site is not immediately visible, as large sections of the fortress lie buried beneath the modern city. The Rheinische Landesmuseum Bonn displays finds from the location and helps visitors understand what once stood there.

The fort's Via principalis, its main cross street, corresponds almost exactly to the course of today's Römerstraße in Bonn. The camp lay between today's Augustusring and Rosental, with its centre at the crossing of the Roman road and Nordstraße/Badener Straße.