Gare Montparnasse

Yes, See Consignes Automatique at https://www.gares-sncf.com/fr/gare/frpmo/paris-montparnasse/services

http://parisbytrain.com/paris-train-station-and-airport-luggage-storage/

Paris By Train is a long-running blog with very useful guides to the main train stations, elaborately explained with photos. This link covers baggage storage.

It can be tricky to find - or maybe my husband and I are just dunces...there was a sign to the upper floor - couldn't find it then saw a sign that it was on a floor down...sigh...I remember my frustrated facebook status saying - we made it to Paris, maybe we'll see some of it when we find the %^#& luggage storage...lol. We were able to fit our two carry ons and two personal items in one of the big ones...just barely. There is security to go thru...and if you don't have coin for the locker, there is a machine you can put your credit card in and it'll give you change (we were plum out after leaving Belgium!). Cost was 7.50 euro for the large locker. There are three sizes if memory serves...

Thanks everyone.
Over & out.

As I recall, when orienting yourself waling in through the main door of the station, the luggage storage is in the far front left corner of the building downstairs. You need change for the lockers, but when we were there a maned booth inside the locker area would provide change.