The law, passed by the legislature last year, will require people under 18 to wear helmets while riding a bicycle or risk a $50 fine.
Kids under 14, however, are exempt from the fine, as are teens who don't wear helmets for religious reasons.
First-time offenders can also avoid the fine by taking an online safety quiz.
Doctors Manitoba, the provincial medical association, has long pushed for a law that would require adults to wear helmets as well.
The group says helmets for all cyclists would reduce the number of severe head injuries suffered every year.