The Malbork economy was one of the most profitable royal economies. Based on the 1590 parliamentary constitution, it was included in the so-called royal table estates, intended solely to meet the needs of the king and the court treasury. The seat of the treasurer’s office was the royal castle in Malbork. An extensive collection of documentation of the treasury has been preserved, including, among other things, contracts between monarchs and successive treasurers. This paper presents a hitherto unpublished document of the contract concluded by King John Casimir with the starost of Sztum, Sigismund Guldenstern, in 1651. It is a valuable addition to previously published sources on the history of the Malbork economy in the 17th century.