Mieszko II the Fat (Polish: Mieszko II Otyły) (c. 1220 – 22 October 1246) was a Duke of Opole-Racibórz from 1230 until his death, and Duke of Kalisz-Wieluń during 1234–1239 (with his brother as co-ruler).
He was the eldest son of Duke Casimir I of Opole by his wife Viola, probably a Bulgarian lady.
At the time of his father's death in 1230, Mieszko II and his younger brother Władysław were still minors; for this, the regency of the Duchy was taken by their mother and Henry I the Bearded, Duke of Wrocław. Following her late husband's politics, the Dowager Duchess Viola led a policy of close cooperation with the Church. In 1233, and probably with the consent of the Duke of Wrocław, was issued a bull by Pope Gregory IX, under which the tutelage of the young Dukes was removed from their mother and granted to the Archbishop of Gniezno and the Bishops of Wroclaw and Olomouc. A year later, to calm the erupted rebellion against this decision, Henry I the Bearded give Kalisz and Wieluń to Casimir I's sons, taking the full control over Opole-Racibórz, but without denying their hereditary rights.
In 1238, Henry I the Bearded died and was succeeded by his son Henry II the Pious, who also assumed the regency over Opole-Racibórz. Shortly after, Mieszko II began to claim the government of their lands. It is unknown exactly when Mieszko II took over the rule over Opole-Racibórz, but this has been probably in late 1238 or early 1239 (who is more likely). One of the first politic decisions of Mieszko II was his marriage with Judith (b. 1222/25 – d. 4 December 1257/65?), daughter of Duke Konrad I of Masovia, who gave to the Duke of Opole-Racibórz greater autonomy in his rule. Mieszko II's mother Viola and brother Władysław are expected to remain in Kalisz, which held power under the guidance of the Duke of Wrocław.