This page lists various jazz jam sessions that are happening in Montreal. Note that this only lists events that are "officially" advertised as jam sessions. There are many events around town that are not officially jam sessions but, if you know the musicians involved, you can definitely sit in with the band on a tune or two. Such events are not included here. Also note that non-jazz music jam sessions are not listed here.
Also, during the Montreal Jazz Festival there are jam sessions all over town every single night- these are not listed here either.
If you are new to jam sessions and would like to participate, I recommend going as a spectator first. See whether or not you feel comfortable with the style of music, the musicians involved, the venue, etc. If you do and wish to play and have never done so before, I would recommend reading a jam session etiquette guide.
Atelier de Jazz avec Roberto Murray, Serge Lavoie, Simon Pagé, 10:30PM
More of an informal weekly gathering of jazz musicians, this jam session sometimes does not have a full house band and sometimes moves into free improvisation territory.
Grumpy's Jazz Jam with Éric Thibodeau, 10:30PM
1242 Bishop (below St-Catherine)