Karl II, Prince of Isenburg-Büdingen in Birstein (full name: Karl Viktor Amadeus Wolfgang Kasimir Adolf Bodo) (29 July 1838 – 2 April 1899) was head of the mediatised German house of Isenburg and Büdingen.

Karl was born in Birstein, Hesse, as the second child of Victor Alexander, Prince of Isenburg-Büdingen, son of Karl, last sovereign Prince of Isenburg, and his wife, Princess Maria of Löwenstein-Wertheim-Rosenberg.

[1] After the early death of his father in 1843, Karl was placed in the charge of his uncle, Wolfgang Ernst III, Prince of Isenburg-Birstein, who had him raised in his Protestant denomination, but he was exposed to the Catholic faith by his mother and in 1861, converted to Catholicism.

On May 31, 1865, at the age of twenty-seven, he married Archduchess Maria Luisa of Austria (1845–1917), daughter Leopold II, Grand Duke of Tuscany and his second wife, Princess Maria Antonia of the Two Sicilies.

In 1866, after the death of his uncle, Karl succeeded him as Prince of Isenburg-Birstein.