Nicholas I may refer to:

• Pope Nicholas I (c. 800–867), or Nicholas the Great
• Nicholas Mystikos (852–925), Patriarch Nicholas I of Constantinople
• Nicholas I (bishop of the Isles) (fl. 1147–1152), Bishop-elect of the Isles
• Nicholas I, Lord of Mecklenburg (died 1200)
• Nicholas I of Transylvania (died after 1203), voivode of Transylvania
• Nicholas I (bishop of Schleswig) (died 1233)
• Patriarch Nicholas I of Alexandria, Greek Patriarch of Alexandria between 1210 and 1243
• Nicholas I, Duke of Troppau (c. 1255–1318), natural son of king Ottokar II of Bohemia, became Duke of Troppau in Silesia
• Nicholas I, Lord of Rostock (died 1314)
• Nicholas I, Count of Tecklenburg (died 1367)
• Nicholas I Garai (died 1386), chief governor of Bratislava, palatine to the King of Hungary
• Nicholas I of Opole (c. 1424–1476)
• Nicholas I, Duke of Lorraine (1448–1473)
• Nicolaus I Bernoulli (1687–1759); Swiss mathematician
• Nicholas I, Prince Esterházy (1714–1790), Hungarian prince
• Nicholas I of Russia (1796–1855), Emperor of Russia and King of Poland
• Nikola I Petrović-Njegoš (1841–1921), King of Montenegro

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Nicholas I

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• Niccolò I (disambiguation)
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