Johnny Manziel -- Boozin' at Nightclub ... Hours After Case Sent To Grand Jury

Johnny Manziel spent his Thursday night pounding champagne in a hot Miami nightclub -- a bizarre way to spend the night hours after officials announced his domestic violence case was going to a grand jury.

TMZ Sports spoke with multiple witnesses (including a staff member) -- and obtained video -- showing the embattled QB drinking straight from what appears to be a champagne bottle at Story nightclub in Miami.

Manziel tried to go semi-incognito in a big hat and sunglasses ... but it's obvious it's Manziel.

Hours before Johnny hit the club, the Dallas PD announced the investigation into allegations Manziel struck Colleen Crowley on Jan. 30th was sent to the grand jury, which will decide if charges should be filed against him.

There's a lot at stake for Johnny ... not just his NFL career, but jail time is on the table if he's ultimately convicted of misdemeanor assault/domestic violence.

Maybe the move to party is to show people he's not worried -- but it doesn't seem like the best PR move to us, considering his past ... a past that includes a stint in a treatment center (though he never said what the treatment was for).