Nick Reiner Hires Defense Attorney Alan Jackson in Parents' Murder Case

Nick Reiner is calling in the heavies after being arrested for allegedly murdering his famous parents ... because Alan Jackson tells us Nick hired him as his defense attorney.

Alan is one of the most well known criminal defense attorneys out there, and he's no stranger to high-profile cases like this one ... he recently got Karen Read acquitted of murder in Massachusetts.

As you know ... Nick was booked on murder charges after Rob and Michele Reiner were found dead Sunday in their Brentwood home ... and he's being held without bail.

We saw Alan heading into a Los Angeles courthouse Tuesday for some legal proceedings related to Nick's case ... so it's game on.

TMZ broke the story ... Nick got into a heated argument with Rob at Conan O'Brien's Christmas party Saturday night, loud enough for others to overhear. Rob and Michele left the party after the fight.

Around 4 AM Sunday morning, Nick checked into a Santa Monica hotel ... and left a trail of blood behind ... sources with direct knowledge told TMZ.

Nick's been open about struggles with substance abuse, cycling through rehab centers, and being homeless at some points ... and it will be interesting to see what kind of defense he presents.