Nicholas I (Czech: Mikuláš I. Opavský) (c. 1255 – 25 July 1318) was the natural son of Bohemian king Ottokar II Přemysl and his mistress Agnes of Kuenring. In 1269 he became Duke of Opava (at modern day Opava, Czech Republic) and thereby the progenitor of the Silesian cadet branch of the Přemyslid dynasty that lasted until 1521.
He was legitimated by his father with the consent of Pope Alexander IV and raised at the Prague court. As his half-brother Wenceslaus II was designated to succeed his father on the Bohemian throne, Nicholas in compensation received Troppau, then a part of the Moravian march. He supported his father in the 1278 Battle on the Marchfeld, was captured by Hungarian forces, but regained his duchy from the victorious German king Rudolf of Habsburg. His rule was however challenged by Ottokar's widow Kunigunda of Halych, who had retired to Hradec nad Moravicí.
In 1283 Nicholas married King Rudolf's niece Adelheid. They had three sons:
Nicholas II, Duke of Troppau
Wenceslaus of Opava
Johann (died 1325).
Nicholas retained the Duchy of Opava after the last Přemyslid ruler of Bohemia, King Wenceslaus III was killed in 1306. The Bohemian Crown however passed to Duke Henry of Carinthia and Nicholas had to accept the pledge of his duchy in favour of the Silesian Piast duke Bolesław III the Generous of Legnica, the husband of King Wenceslaus' III sister Margaret. King Henry's successor John of Bohemia however redeemed the pawn and in 1318 re-installed Nicholas's son Nicholas II as duke.