| En Name: | Onguday |
| Ru Name: | Онгудай |
| Loc Name1: | Оҥдой |
| Loc Lang1: | Altay |
| Coordinates: | 50.75°N 94°W |
| Map Label Position: | right |
| Image Coa: | Coat of Arms of Ongudaysky District.svg |
| Federal Subject: | Altai Republic |
| Adm District Jur: | Ongudaysky District |
| Adm Selsoviet Type: | Selsoviet |
| Adm Selsoviet Jur: | Ongudaysky |
| Adm Selsoviet Jur Ref: | [1] |
| Adm Ctr Of1: | Ongudaysky District |
| Adm Ctr Of2: | Ongudaysky Selsoviet |
| Inhabloc Cat: | Rural locality |
| Inhabloc Type: | Selo |
| Mun Data As Of: | March 2015 |
| Mun District Jur: | Ongudaysky Municipal District |
| Mun District Jur Ref: | [2] |
| Rural Settlement Jur: | Ongudayskoye Rural Settlement |
| Mun Admctr Of1: | Ongudaysky Municipal District |
| Mun Admctr Of2: | Ongudayskoye Rural Settlement |
| Pop 2010Census: | 5655 |
| Date: | September 2018 |
| Onguday population | |
| Label1: | 2010 Census |
| Data1: | 5,655 |
| Label2: | 2002 Census |
| Data2: | 5,376 |
| Label3: | 1989 Census |
| Data3: | 5,399 |
| Label4: | 1979 Census |
| Data4: | 4,472 |
Onguday (Russian: Онгуда́й; Southern Altai: Оҥдой, Oñdoy) is a rural locality (a selo) and the administrative center of Ongudaysky District in the Altai Republic, Russia, located on the Ursul River, 210km (130miles) from Gorno-Altaysk, the capital of the republic. As of the 2010 Census, its population was 5,655, consisting mainly of the Altai people and Russians.
Local name Ongdoy (from mongolian onge — "cave, cavity", doy — possession affix;literally — "having cave, cavities"
It was founded in 1626. In 1856-1857, the camp-settlement of the Altai spiritual mission of the Russian Orthodox Church was founded. In 1860, the first school was opened, for which a building was built in 1881. In 1879, the Assumption Church was built and consecrated in 1881. In 1908 the Church of St. Innocent of Irkutsk was built. In 1910, about 800 people lived in the village.
Onguday has a monsoon-influenced humid continental climate (Köppen climate classification Dwb) with warm, humid summers and severely cold, dry winters. The warmest month of the year is July, when the average temperature is, the highest temperature recorded being . The coldest month of the year is January, when the average temperature is, with the record low of .