The article discusses the seals of the rulers who reigned in the Principality of Niemodlin in two periods of its history. The first, covering the years 1313–1382, corresponds to the times when a separate line of the Piasts of Niemodlin, represented by Bolesław Pierworodny and his two sons Wacław and Henryk, existed. The second, falling in the years 1382–1455, marks the reign of their relatives from Opole. It started with the joint reign of Jan Kropidło, Bolesław IV and Bernard who were brothers. In 1400, Bernard started his independent reign. A total of 15 types of seals have survived the times covered by the study, most of which, namely 11, are seals of arms depicting an image of the Opole eagle on a shield inclined to the right or set straight ahead. Jewelled helmets are always adorned with those with tilted shields. Before 1382, plumes had been attached to helmets instead of the Opole eagles. The same eagle as a jewel also appears on three helmet-and-jewel seals, two of which belonged to Boleslaw IV and one to Bernard. The only type of portrait seal, exposing the bust of the woman of property holding in her hands the symbols manifesting the origin and princessly dignity, namely the family coat of arms and the helmet with the Opole eagle as a jewel, was used by Princess Eufemia, the wife of Bolesław Pierworodny. In the period up to 1382, each Duke of Niemodlin had only had one great seal, while Boleslaw and Henry had also used the second seal as a minor seal and a counterseal. A separate counterseal was additionally at Bolesław’s disposal. He was also the only one who in the early period of his reign replaced the great seal and introduced a more representative seal instead of the seal matrix. The later rulers of Niemodlin, with the exception of Jan Kropidło who had many types of seals at his disposal due to his episcopal dignities, did not have an elaborate sphragistic system. Boleslaw IV used two larger seals but at a certain point he used only one, which entailed the withdrawal of the second seal from use. He also used a smaller seal as a counterseal. However, it is impossible to clearly determine the legal value of Bernard’s three seals. After all, all three were used as the main seals. The oldest seal, which could also be regarded as a minor seal, was the only seal Bolesław IV possessed in the first two decades of his reign. He also used it to close his letters. Bolesław’s helmet-and-jewel seals showed a crowned eagle for the first time in the heraldry of the Opole Piasts. Bernard’s helmet-and-jewel seal and his younger seal of arms also showed crowned eagles. From then on, it settled permanently among other rulers of the Opole line. The Opole eagle itself, displayed on a shield or as a helmet jewel, replaced the traditional image of a ruler and became its synonym.