Western Ontario is a region of Ontario centred on London, Ontario. It extends north to south from Lake Huron to Lake Erie, and east to west roughly from Kitchener to Windsor. Other cities in the region are Ingersoll, St. Thomas, Stratford, and Woodstock. Western Ontario is not strictly speaking in the west of Ontario, but it is in the west of the part of the original province (the south) which was first settled.

The three westernmost counties (Essex, Lambton and Chatham-Kent) are also commonly grouped as the region of Southwestern Ontario.

It is a prosperous agricultural region whose chief crops are tobacco, sweetcorn, soybean, and winter wheat. Dairy and beef farming and the breeding and training of standardbred horses are also important industries.

Much of Western Ontario was part of the Talbot Settlement, and the region has benefited from the Settlement's facilitation of agriculture and of trade in general.

The University of Western Ontario is in London.