Theodora Porphyrogenita (Greek: Θεοδώρα Πορφυρογέννητη, Theodōra Porphyrogenítē; 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 Byzantine Empire for almost 200 years.

Theodora became involved in political matters only late into her life. Her father Constantine VIII was co-ruler of the Byzantine Empire for 63 years then sole emperor from 1025 to 1028. After he died, his older daughter Zoë co-ruled with her husbands, then her adopted son 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 Zoë.

After 65 days Zoë married again, to Constantine IX, who assumed the imperial responsibilities. Theodora seemingly retired to a convent after Zoë'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.