During the Ottoman-Russian War of 1768-74, Necati Efendi (d. after 1776) served as the secretary to Silahdar Ibrahim Pasha, the Ottoman commander-in-chief of the Crimea. As a result of military events, he was taken prisoner with some 40 other officials: he spent almost four years in Russian captivity (1771-75), only to regain his freedom after the Peace of Küçük Kaynarca in July 1774. This paper relies on Necati Efendi’s captivity narrative, available in a number of manuscripts. While there are a number of embassy reports or comparable records from 18th-century Russia, Necati’s memoirs are the only known captivity narrative. Necati describes both the itinerary of the Ottoman group from Crimea to St Petersburg and their captivity. A primary source of immense significance for our knowledge about the reign of Catherine II and her political relations with the Ottoman Empire, this first-person narrative offers not only descriptions of contemporary St Petersburg, Catherine’s II palace, and diplomatic encounters, but also provides glimpses into the lives of Ottoman captives. Drawing on the concept of semiosphere coined by Yuri Lotman in 1984, I argue that the mobility of knowledge in Necati’s account did not merely take place between Ottoman and Russian borders and as a one-dimensional process of adoption. Rather, the mobility of knowledge in this text can be observed in intertwined spheres and on multiple levels concerning space, time, and in the text itself, i.e. on a dynamic continuum of semiotic systems. Textual representation is a part of this dynamic process of the adoption of knowledge, since intellectual reflections on the real sphere also influences the textual sphere. Thus, Necati Efendi’s experience during his captivity characterized his cognitive understanding of his own world and a foreign culture. In the contemporary Transottoman context of Istanbul and St Petersburg, this analysis allows us to understand cultural dynamics not simply as a static or binary process, but as a continuum in which borders and the peripheries of borders entangle and interconnect to one another in semiospheres. This captivity narrative reveals cognitive entanglements in this context and the form that process take as spaces of new knowledge emerge in the text. Thus, a semiospheric understanding of borders offers the opportunity to revisit the nature of inter-imperial relations against the backdrop of military conflicts during a transitional period in Ottoman politics and modernization in the late 18th century.