Melissa McCarthy embraced her inner criminal on the "Law & Order: SVU" set ... donning a disguise while pulling up to guest star on the long-running hit show.

The star shared a video on Instagram Thursday night shortly after her cameo on the show aired ... which kicks off with her saying she wants to surprise Mariska Hargitay -- going so far as getting a rubber mask to hide her identity.

It's certainly not a "Mission Impossible" level identity concealer -- but it apparently did the trick -- because MH didn't know MM was there until she walked onto the set in full costume.

Check out the clip ... it takes Hargitay a few seconds to recognize McCarthy ... but she goes berserk when she does, swearing in surprise and wrapping her fellow star up in a big hug.

McCarthy -- dressed like a pro wrestler -- cracks up at her pal's reaction and tells her she's there to cameo ... and, Hargitay jokes they'll need to rewrite the script for the star.

Of course, these two stars couldn't help but play around ... and Hargitay slapped a pair of cuffs on McCarthy in a hilarious end to the vid.

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In case you haven't seen the scene in 'SVU' yet ... ya gotta check it out ... McCarthy's character gets real flirty with Hargitay's Olivia Benson, including complimenting her "gams."

Even your favorite star still gets starstruck!