Frederick Casimir of Cieszyn also known as of Fryštát (Polish: Fryderyk Kazimierz Cieszyński or Frysztacki; German: Friedrich Kasimir von Teschen-Freystadt; b. December 1541/January 1542 - d. 4 May 1571), was a Polish prince member of the House of Piast in the Cieszyn branch and ruler over Fryštát, Skoczów (both since 1560), and Bielsko (since 1565).

However, this changed in 1567 when, after his mother's death, his father remarried with Sidonie Katharina of Saxe-Lauenburg and three years later, in 1570, she gave birth to her first son, Christian August.

The premature death of his new half-brother within eleven months placed Frederick Casimir again as the only heir of Cieszyn.

Although he did not have a high income, Frederick Casimir ran a sumptuous lifestyle, as in a short period he led his lands on bankruptcy.

The total amount of his debts, after a revision of a special Imperial Committee was the astronomical sum of 244,000 talers.