The article discusses the figure of Jan Piotrowski, a forgotten and underrated Old Polish diarist, native of the Wielkopolska Province, priest and secretary to King Stefan Batory of Poland. Having adopted the pen name “A friend at the royal court” Piotrowski wrote secret reports about various courtly events to Polish aristocrat and politician Crown Marshal Andrzej Opaliński, who lived outside the Polish royal court in order to provide him with information helpful in his political activities. His letters set in chronological order constitute a systematic diary and are a broad and vivid reflection of Polish politics, diplomacy and military culture of the last quarter of the 16th century. Many of Piotrowski’s letters remain currently in the Kórnik Library of the Polish Academy of Sciences. Apart from Piotrowski’s well-known letters published under the pen name of “A friend at the royal court” the author of the article examines his unpublished letters as well as hypothesizes about Piotrowski’s authorship of a diary concerning Polish-Austrian negotiations – the so-called Będzin settlement. The settlement was a failed attempt to put Maximilian Archduke of Austria on the Polish throne. The author of the article proves that the hitherto unpublished diary existing in two copies in the Kórnik Library was written by Jan Piotrowski. This fact not only expands Piotrowski’s epistolary legacy and contributes to the Old Polish tradition of writing political diaries, but it also sheds light on Polish-Austrian relations, which have been largely neglected in works of both Polish and Austrian historians.