The Duchy of Gaeta (Latin: Ducatus Caietae) was an early medieval state centered on the coastal South Italian city of Gaeta. It began in the early ninth century as the local community began to grow autonomous as Byzantine power lagged in the Mediterranean and the peninsula due to Lombard and Saracen incursions.

The primary source for the history of Gaeta during its ducal period is the Codex Caietanus, a collection of charters preserving Gaetan history better and in greater detail than that of its neighbouring coastal states: Naples, Amalfi, and Sorrento. In 778, it was the headquarters from which the patrician of Sicily directed the campaign against the Saracen invaders of Campania.

The first consul of Gaeta, Constantine, who associated his son Marinus with him, was a Byzantine agent and a vassal of Andrew II of Naples. Constantine defended the city from the ravages of Muslim pirates and fortified it, building outlying castles as well. He was removed, probably violently, by Docibilis I, who established a dynasty and made Gaeta de facto independent.

The Docibilian dynasts regularly worked to advance Gaetan interests through alliance with whatever power was most capable of such at the time. They joined forces with the Saracens against their Christian neighbours and with the Pope against the Muslim pirates at the Battle of Ostia. They constructed a massive palace and greatly increased the city's prestige and wealth. The Gaetans remained nominally Byzantine in allegiance until the mid tenth century, fighting under their banner at the Battle of the Garigliano. The chief success of the Docibilians lay, however, in extracting Gaeta from the Ducatus Neapolitanus.

It was Docibilis II (died 954) who first took the title of dux or duke (933). Docibilis saw Gaeta at its zenith but began the process whereby it was chiefly weakened. He gave Fondi to his second son Marinus with the equivalent title of duke and set a precedent for the partitioning of the Gaetan duchy and its encastellation, which corroded ducal authority over time.

In 962, Gaeta put itself under Pandulf Ironhead, the Lombard prince of Capua. In 963, however, only the municipal rulers appeared in the charters. In 976, the Holy Roman Emperor, Otto II, and the pope were the recognised suzerains of Gaeta. A complete revolution had occurred since the assumption of the ducal title and the Western Emperor had replaced the Eastern as overlord.

Gaeta declined in importance in the late tenth and early eleventh centuries. In 1012, a succession crisis weakened it further. John IV died, leaving one son by his wife Sichelgaita, a sister of Sergius IV of Naples. This son, John V, ruled under the disputed regency of his grandmother Emilia. His uncle Leo I usurped the duchy only to be removed in a few short months and his other uncle, Leo II, fought over the regency with Emilia. It wasn't until 1025 that the situation was settled. After that, John V sheltered the fleeing Sergius of Naples and aided him in retaking his city with Norman assistance. For this, John V earned the enmity of Pandulf IV of Capua and his duchy was conquered in 1032. The local dynasty, descended from Docibilis, would never recover its duchy.

Gaeta was conquered by the Lombards in 1032. In 1038, the conqueror, Pandulf of Capua, was deposed and replaced by Guaimar IV of Salerno. Guaimar did not reign personally for long before appointing the chiefest of his Norman mercenaries, Ranulf Drengot, as duke. On Ranulf's death, however, the Gaetans elected their own Lombard candidate, Atenulf, Count of Aquino.