Chris Brown SNEAKS Into Jail!

Chris Brown pulled a fast one Monday ... turning himself in at an L.A. jail for his hit-and-run case -- a day ahead of schedule.

TMZ has learned ... Brown showed up at the Van Nuys jail at around 2 PM. We're told his lawyer, Mark Geragos, engineered it all and the purpose was to make it happen without any photogs present.

Brown was booked and released on his own recognizance -- no bail. We're told the whole thing took around 45 minutes.

Now here's what's interesting ... Just a few hours earlier, there was a gaggle of photogs at the courthouse which is connected to the jail, because 50 Cent made a court appearance. When the cameras were gone, Chris appeared.

Sneaky, Geragos.