Ernest or Ernst Casimir may refer to:
Ernest Casimir, Count of Nassau-Weilburg (1607–1655), founder of the younger line of Nassau-Weilburg Ernest Casimir I, Count of Nassau-Dietz (1573–1632) Prince Ernest Casimir of the Netherlands (May 1822 – August or October 1822) Ernst Casimir I, 1st Prince of Ysenburg and Büdingen (1781–1852) Ernst Casimir II, Prince of Ysenburg and Büdingen (1806–1861)
See also Ernest Casimir-Lambert (1897–1931), Belgian bobsledder