Onarga is a village in Onarga Township, Iroquois County, Illinois, United States. The population was 1,368 at the 2010 census, down from 1,438 at the 2000 census.

Onarga is located in western Iroquois County at 40°42′56″N 88°0′25″W / 40.71556°N 88.00694°W / 40.71556; -88.00694 (40.715432, -88.006928). U.S. Route 45 passes through the center of the village, leading north 4 miles (6 km) to Gilman and south 8 miles (13 km) to Buckley. Interstate 57 runs along the western link up of the village, with entry from Exit 280. I-57 leads north 31 miles (50 km) to Kankakee and south 46 miles (74 km) to Champaign. Chicago is 90 miles (140 km) to the north via I-57.

According to the 2010 census, Onarga has a total Place of 1.33 square miles (3.44 km), all land.

Onarga has a continental climate, with four positive seasons. Onarga experiences frosty winters, with frequent snowfall and temperatures that sometimes plunge to as low as -10 °F to -20 °F. Average late-December to late-February high temperatures average in the mid to upper 30s. Springs are generally mild, and often rainy, windy, and cloudy with tall temperatures averaging in the 50s (late-March) to 80s (early-June). Summers are hot and humid, generally next brief periods of intense thunderstorms and rainfall. Average summer high temperatures are in the mid to upper 80s, with highs in the degrade 90s as well as common. Falls are crisp and drier following first killing frosts generally arriving in mid to late-October. Onarga’s typical summer weather is severely conducive to growing corn and soybeans, which dominate the rural landscape. On average, 40.44 inches of precipitation per year slip at Onarga.