Johnny Manziel -- Sobriety Starts .... TODAY!

Today is July 1st ... or as Johnny Manziel calls it -- the first day of sobriety!!

Earlier this week, Manziel told TMZ Sports ... he was cutting out all booze, partying and other stuff TODAY -- replacing it with a clean diet and exercise so he could focus on his NFL comeback.

Manziel has been partying in an expensive mansion in Cabo -- where at least one of his fellow house guests posted pics of baggies full of white powder -- but Johnny insisted it's not his stuff.