Nick Stahl's 2013 is off to a pleasurable start ... the "Terminator 3" actor won't be charged for allegedly whacking off in a Hollywood XXX shop, TMZ has learned.

We broke the story ... Nick was arrested back in December for "lewd conduct" -- a misdemeanor -- after cops made a routine check at an adult book store and allegedly found the troubled actor "touching himself" while watching a porno.

But the Los Angeles City Attorney's office is throwing Nick a bone -- declining to prosecute him due to "insufficient evidence."

It's a significant bit of good news for a guy who's had a long streak of bad ... which was written all over Nick's face right after the arrest.