Toronto Public Health says a 68-year-old man contracted the virus in mid-July but was never admitted to hospital for it.
The agency says he is recovering at home.
West Nile is transmitted to humans mainly through mosquito bites, but health officials say the risk of being infected is low and most people who are don't get sick.
Officials say there were 94 human cases of the virus in Toronto last year, up from 28 cases in 2011.
They're urging residents to try to ward off mosquitoes by wearing light-coloured, long-sleeved clothes and using bug repellent.