| Leo Diogenes | |
| Emperor of the Romans | |
| Succession: | Byzantine co-emperor |
| Reign: | 1069 – November 1071 |
| Father: | Romanos IV Diogenes |
| Mother: | Eudokia Makrembolitissa |
| Birth Date: | 1069 |
| Death Date: | 14 August 1087[1] (aged 18) |
| Death Place: | Dristra |
| Reg-Type: | Senior emperor |
| Regent2: | Nikephoros (1069–1071) Michael VII (1071–1078) Konstantios (1060–1078) Andronikos (1068–1070s) |
| Regent: | Romanos IV Diogenes |
Leo Diogenes (Greek, Modern (1453-);: Λέων Διογένης, Leōn Diogenes, 1069 – 14 August 1087), styled as porphyrogenitus, was the son of Byzantine Emperor Romanos IV Diogenes and Eudokia Makrembolitissa. Likely crowned co-emperor during his father's reign, he later served in the armies of Emperor Alexios I Komnenos. He does not appear on any of Romanos' coins, although there is at least one letter that refers to him as emperor (basileus).[2] Anna Komnene notes that he and his brother Nikephoros both wore the diadem and tzangion (red sandals) usually reserved to emperors.[3]
Leo's birth is not recorded in sources. He had a brother, Nikephoros Diogenes, whose birth is not recorded either.[4] Romanos IV Diogenes and Eudokia Makrembolitissa married on 1 January 1068, meaning they were born in late 1068 or early 1069 at the earliest, probably as twins.
Although elevated to the rank of co-emperor on their birth, they were both was banished to a monastery along with their mother after the fall of Romanos. Here they remained until the accession of Alexios I Komnenos in 1081, who took them and raised them like his own sons.
According to Anna Komnene’s account, Leo was a committed supporter of Alexios, who urged him not only to confront the Norman invaders early in his reign,[5] but also the Pechenegs who had invaded the empire from beyond the Danube.[6] During one of the pitched battles against the Pechenegs, Leo allowed himself to be drawn away from the emperor's side, and as he approached the wagons of the enemy, he was struck down and died on the field of battle.[7]
In 1095 an impostor of Leo, Pseudo-Diogenes, convinced the Cuman chieftains Boniak and Tugorkan to invade the Byzantine Empire, dethrone Alexios, install himself as emperor. The Cumans occupied Paristrion before being repulsed by Byzantine forces, led by Alexios.[8] [9]