| Type: | municipality |
| Official Name: | Castrillo de Onielo |
| Subdivision Name3: | Castrillo de Onielo |
| Area Total Km2: | 40 |
| Population Density Km2: | auto |
| Utc Offset: | +1 |
| Utc Offset Dst: | +2 |
Castrillo de Onielo is a municipality located in the province of Palencia, Castile and León, Spain. According to the 2018 census (INE), the municipality has a population of 101 inhabitants.