Jackson’s representatives have chosen not to give out any further information except saying that the star had been ill but was getting better now.
The Montreal concert would have been Jackson's first performance there for seven years.
Boston and Philadelphia concerts on Wednesday and Thursday were also postponed, but at present there are no details about whether they will be rescheduled or what ticket holders should do.
However, Jackson is due to start her North American tour again on Saturday.
Janet Jackson has just been in the news for announcing that that she wants to part company with record label Island Def Jam after only14 months.
