| Official Name: | Onilahy |
| Other Name: | Ionilahy |
| Subdivision Type: | Country |
| Subdivision Type1: | Region |
| Subdivision Name1: | Fitovinany |
| Subdivision Type2: | District |
| Subdivision Name2: | Manakara |
| Population As Of: | 2001 |
| Population Total: | 5000 |
| Population Blank1 Title: | Ethnicities |
| Population Blank2 Title: | Religions |
| Timezone: | EAT |
| Utc Offset: | 3 |
| Coordinates: | -21.7167°N 85°W |
| Elevation M: | 211 |
| Postal Code Type: | Postal code |
| Postal Code: | 316 |
Onilahy (or Ionilahy) is a rural municipality in Madagascar. It belongs to the Manakara-Atsimo District, which is a part of Fitovinany. The population of the commune was estimated to be approximately 5,000 in 2001 commune census.
Only primary schooling is available. It is also a site of industrial-scale mining. Farming and raising livestock provides employment for 40% and 40% of the working population. The most important crops are cassava and rice, while other important agricultural products are bananas and oranges. Industry and services provide employment for 19.7% and 0.3% of the population, respectively.[1]
Ionilahy is a station on the Fianarantsoa-Côte Est railway.