Casimir I of Kuyavia (Polish: Kazimierz I kujawski; c. 1211 – 14 December 1267) was a Polish prince and a member of the House of Piast. He was Duke of Kuyavia after 1233, ruler over Ląd from 1239–1261, ruler over Wyszogród after 1242, Duke of Sieradz from 1247–1261, Duke of Łęczyca after 1247, and Duke of Dobrzyń after 1248.
He was the second son of Konrad I of Masovia and his wife Agafia of Rus. He was probably named after his grandfather, Casimir II the Just.
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Life
Casimir I received Kujawy (however, without Dobrzyń) from his father in 1233. In 1239, he would enlarge his domains with the castellany of Ląd, which he received as the dowry of his second wife. In subsequent years, Casimir I actively supported his father's turbulent politics, which brought him in 1242 to conquer the district of Wyszogród from the rulers of Gdańsk.
Konrad I died on 31 August 1247. According to his will, most of Masovia passed to his oldest son, Bolesław I, and the youngest of his sons, Siemowit I, received Sieradz-Łęczyca. Dissatisfied with his part of the paternal inheritance, Casimir I attacked his brothers' lands while they were in Płock at the funeral of their father, and was able to conquer Siemowit I's inheritance.
The next geopolitical change in Masovia came in early 1248, when Bolesław I died unexpectedly and without issue. In his will, he left all his domains to his brother Siemowit I. With this opportunity, Casimir I was able to take advantage of the confusion after his older brother's death and capture Dobrzyń.