Zōstē patrikía (Greek: ζωστὴ πατρικία) was a Byzantine reserved exclusively for the woman who was the chief attendant and assistant to the Empress. A very high title, its holder ranked as the first woman after the Empress herself in the imperial court. The title is attested from the 9th century until the 12th century, but only a handful of its holders are known.
The title means "girded lady-patrician", often translated into English as "Mistress of the Robes", and was used for high-ranking court ladies who were attached to the Byzantine empresses as their ladies of honour. Its origin or date of institution are unclear. Disregarding a clearly anachronistic reference to Antonina, the wife of the great 6th-century general Belisarius, as being a zostē patrikia, the title is first attested in circa 830 for Theoktiste, the mother of Empress Theodora. The title is last attested in literary sources (the Skylitzes Chronicle) in 1018, when it was conferred to Maria, the former Empress of Bulgaria, and finally in a series of lead seals dated to the late 11th century (see below). It disappears thereafter, along with many other titles of the middle Byzantine period, following the reforms of Emperor Alexios I Komnenos (r. 1081–1118).
In Philotheos's Klētorologion of 899, the dignity of the zostē patrikia is placed very high in the imperial order of precedence, coming before the magistros and after the kouropalatēs. Her exceptional status is further illustrated by the fact that she was one of only six dignitaries who dined at the imperial table—along with the Patriarch of Constantinople, the Caesar, the nōbelissimos, the kouropalatēs and the basileopatōr—and by the prominent role she played in imperial ceremonies, especially those revolving around the Empress, such as the coronation of an empress or the birth of a child. The zōstē patrikia functioned as the chief attendant to the empress (to whom she was usually related) and the head of the women's court (the sekreton tōn gynaikōn), which consisted mostly of the wives of high-ranking officials. Indeed, hers was the only specifically female dignity: other women bore the feminine versions of their husbands' titles. A zōstē patrikia is therefore, in John B. Bury's words, "the only lady who was πατρικία in her own right", and not to be confused with a simple patrikia, who was the spouse or widow of a patrikios. The French scholar Rodolphe Guilland points out that the title itself appears to be a compound one, with the sources sometimes calling it "the zōstē and patrikia", indicating that the noble title of patrikia was added to the court dignity of zōstē. Although it appears that, in common with the other supreme dignities with which it is associated, there was a single holder of the dignity at each time, at the reception of Olga of Kiev, the plural form zōstai is used, indicating the presence of at least two. This may be accounted for by the fact that at times there were several empresses, and that each one must have had a zōstē in her particular service.