Orillia is a city in Ontario, Canada. It is in Simcoe County between Lake Couchiching and Lake Simcoe. Although it is geographically located within Simcoe County, the city is a single-tier municipality. It is part of the Huronia region of Central Ontario. The population in 2021 was 33,411.

It was incorporated as a village in 1867, but the history of what is today the City of Orillia dates back at least several thousand years. Archaeologists have uncovered evidence of fishing by the Huron and Iroquois peoples in the area over 4,000 years ago, and of sites used by Aboriginal peoples for hundreds of years for trading, hunting, and fishing.

Known as the "Sunshine City", the city's large waterfront attracts many tourists to the area every year, as do a number of annual festivals and other cultural attractions. While the area's largest employer is Casino Rama, overall economic activity in Orillia is a mixture of many different industries including manufacturing, government services, customer service and tourism.

Orillia is located on the shores of two connected lakes: Lake Simcoe and Lake Couchiching. Both lakes are part of the Trent-Severn Waterway. Travel north on Lake Couchiching, then through three locks and the only marine railway (Big Chute Marine Railway) still in use in North America leads to Georgian Bay on Lake Huron. Travel south-east across Lake Simcoe, through many locks (including two of the highest hydraulic lift locks in the world, Peterborough Lift Lock and Kirkfield Lift Lock) eventually leads to Lake Ontario. From either of these Great Lakes one can connect to the St. Lawrence and thence to the Atlantic Ocean.