"The Frog Prince; or, Iron Henry" (German: Der Froschkönig oder der eiserne Heinrich, literally "The Frog King or the Iron Henry") is a German fairy tale collected by the Brothers Grimm and published in 1812 in Grimm's Fairy Tales (KHM 1).

[1] It has been postulated by some scholars that parts of the tale may extend back until at least Roman times; an aspect of the story is referred to in Petronius' Satyricon, in which the character Trimalchio remarks, "qui fuit rana nunc est rex" ("The man who was once a frog is now a king").

[4] Folklorist Stith Thompson suggested that the story of the Frog King in the German tradition began with a 13th-century literary tale written in Latin.

In modern versions, the transformation is triggered by the princess kissing the frog (a motif that apparently first appeared in English translations).

[7] The frog prince also has a loyal servant named Henry (or Heinrich) who had three iron bands affixed around his heart to prevent it from breaking from sadness when his master was cursed.

When the frog prince reverts to his human form, Henry's overwhelming happiness causes the bands to break, freeing his heart from its bonds.

[10] In addition, scholars Lutz Röhrich, Waldemar Liungman, and Jurjen van der Kooi noted that, apart from some isolated attestations in the southern part of the continent and in Eastern Europe, variants of the tale were collected in the northern part of Europe, comprising a sort of "core area": Germany, Denmark, Sweden, Finland, Belgium and Netherlands, extending to Ireland and Great Britain.

[11][12][13] However, Röhrich and van der Kooi remarked that the variants collected from oral tradition, even in America, clearly go back to a European original, and Uther argues that they are dependent on the Grimm's tale.

In these tales, a female animal (mouse, cat or frog) helps a prince with three tasks and after marrying him, assumes human form.