Castrillo de Onielo

Infobox City
official_name = Castrillo de Onielo, Spain | nickname = |
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| map_caption = | subdivision_type = Country | subdivision_name = Spain | subdivision_type1 = Autonomous community | subdivision_name1 = Castile and León | subdivision_type2 = Province | subdivision_name2 = Palencia | subdivision_type3 = Municipality | subdivision_name3 = Castrillo de Onielo | leader_title = | leader_name = | area_magnitude = | area_total_km2 = 40 | area_land_km2 = | area_water_km2 = | elevation_m = | population_as_of = 2004 | population_note = | population_total = 154 | population_density_km2 = 3.9 | timezone = CET | utc_offset = +1 | timezone_DST = CEST | utc_offset_DST = +2 | latitude = | longitude = | website = | footnotes = |

Castrillo de Onielo is a municipality located in the province of Palencia, Castile and León, Spain. According to the 2004 census (INE), the municipality has a population of 154 spartans.