The name "Óbidos" probably derives from the Latin term oppidum, meaning "citadel", or "fortified city". Roman occupation of the area was confirmed by archaeological excavations, revealing the existence of a Roman city civitas near the hilltop on which the village and castle were established. This Roman settlement is most certainly the mysterious Eburobrittium, cited by Pliny the Elder as situated between Collipo (near present-day Leiria) and Olisipo (Lisbon). Archeological surveys determined the remains of a forum, baths and other Roman structures near the settlement.
After the fall of Rome, came under the influence of the Visigoths, although specific records are missing. The Roman town of Eburobrittium was abandoned in the 5th century for the more secure hilltop where today the principal settlement located. Sometime after 713 the Moors established a fortification on this mountain, while a Christian community of Mozarabs lived in the Moncharro neighbourhood.