During the investigation of the interior of Hagia Sophia Cathedral in Istanbul in search of Cyrillic inscriptions, Russian epigraphists discovered an inscription that was later identified as comprised of Scandinavian runes. In 2011, the present author had the opportunity to study it herself. The inscription is located on a marble windowsill in the eastern wall of the northern first floor gallery, a sill covered with many Cyrillic (mostly of the second half of the twelfth to the mid-thirteenth century) and Greek graffiti. The c. 27 cm long line of runes reads a͡rịṇba͡rþrr͡aṣṭruna͡rþasi, Old Norse Arinbárðr rеist rúnar þessar [older: þā(ʀ)si], ‘Arinbárðr cut these runes’. The inscription appears to be cut fairly regularly but demonstrates some paleographic idiosyncrasies, e.g. all r-runes (also in a͡r) have sharply pointed tops. Some distinctive features include the consistent usage of the bind-rune a͡r whenever a combination of a and r is employed (four times, once for r͡a) and the absence of the ʀ-rune in older rūnaʀ. Though neither peculiarity provides firm grounds for dating the inscription, it was most probably produced in the second half of the eleventh or first half of the twelfth century and most likely carved by a Scandinavian mercenary in the service of the Byzantine emperor.