Theodora Porphyrogenita (Greek: , Theodra Porphyrogent; c. 980 31 August 1056) was Byzantine Empress from 21 April 1042 to her death on 31 August 1056, and sole ruler from 11 January 1055. She was born into the Macedonian dynasty that ruled the Eastern Roman Empire for almost 200 years.

Theodora became involved in political matters only late into her life. After the death of his father Constantine VIII in 1028, his older daughter Zo co-ruled with her husbands Romanos III, Michael IV and finally Michael V, keeping Theodora closely watched. After two foiled plots, Theodora was exiled to an island monastery in the Sea of Marmara in 1031. A decade later, the people of Constantinople rose against Michael V and insisted that she return to rule alongside her sister Zo.

After 65 days Zo married again, to Constantine IX, who assumed the imperial responsibilities. Theodora seemingly retired to a convent after Zo's death in 1050. When Constantine died, the 74-year-old Theodora returned to the throne despite fierce opposition from court officials and military claimants. For 16 months she ruled as empress in her own right before succumbing to a sudden illness and dying at 76. She was the last ruler of the Macedonian line.

This is a list of Roman and Byzantine empresses. A Roman empress was a woman who was the wife of a Roman emperor, the ruler of the Roman Empire.

The Romans had no single term for the position: Latin and Greek titles such as augusta (Greek αὐγούστα, augoústa, the female form of the honorific augustus, a title derived from the name of the first emperor, Augustus), caesarea (Greek καισᾰ́ρειᾰ, kaisáreia, the female form of the honorific caesar, a title derived from the name of Julius Caesar), βᾰσῐ́λῐσσᾰ (basílissa, the female form of basileus), and αὐτοκράτειρα (autokráteira, Latin autocratrix, the female form of autocrator), were all used.

In the third century, augustae could also receive the titles of māter castrōrum "mother of the castra" and māter patriae "mother of the fatherland". Another title of the Byzantine empresses was εὐσεβέστᾰτη αὐγούστα (eusebéstatē augoústa, meaning "most pious augusta"); they were also called κῡρίᾱ (kūríā, meaning "lady"), or δέσποινα (déspoina, the female form of δεσπότης, despótēs, "despot"). Due to the practice of dividing the Roman empire under different emperors, there were periods when there were more than one Roman empress. All the Roman empresses are listed with some co-empresses. Not all empresses were titled augusta, and not all augustae were empresses since the emperor's sister or mistress could bear that title. Some caesarissas and despoinas that never were empresses are included, since the titles were quite similar to empress; however, in the Eastern Roman Empire these titles are often more equivalent to the modern term "crown princess".