Under Canadian immigration law, detained asylum seekers are held in prison-like facilities although less than 6 % of them are suspected of criminality or dangerousness. We present an overview of the Canadian situation, including data drawn from our recent study on the impact of detention on asylum seekers’ mental health. The study involved 122 adult asylum seekers detained in Immigration Holding Centres in Montreal and Toronto and a comparison group of 66 nondetained asylum seekers. Detained asylum seekers experience a sense of disempowerment and loss of agency linked to the regime of surveillance and strict rules to which they are subjected, as well as to the indeterminate length of their detention. After an average imprisonment of 31 days, detained asylum seekers had a considerably higher level of psychiatric symptoms than did non-detained asylum seekers, although the two groups had undergone equivalent premigration trauma exposure. Under the Protecting Canada’s Immigration System Act (Bill C-31), asylum seekers designated by the Minister, including youth 16 years old and over, will be automatically detained with very limited access to review by an independent tribunal. On the contrary, less restrictive alternative should be considered.