Jennifer Lopez isn't the only one running around town with a hot piece of ass who's half her age ... Marc Anthony's got a hard-bodied 24-year-old of his own ... and last night, he trotted her out in a SKIN-TIGHT sausage-casing of a dress.

43-year-old Anthony has been dating Venezuelan model Shannon De Lima for several months ... and last night, the two strolled in to a private party together in Miami Beach.

With J.Lo still playing chaperone to her ripped boy-toy Casper Smart ... we gotta ask ...

Marc Anthony is making things permanent with his new 24-year-old hottie -- by branding himself with a tattoo of the Statue of Liberty ... a tribute to his GF and his new found "freedom."

Sources close to the singer tell TMZ ... Venezuelan model Shannon De Lima was with Marc when a spot-on replica of Lady Liberty was tatted inside his right wrist on New Year's Eve in Miami. Marc's 17-year-old daughter Ariana (above, right) was also front and center.

We're told "Statue of Liberty" is Marc's pet name for Shannon ... because she "freed" him from his past. While he was at it -- Marc and Ariana also got unspecified matching tats on their outer wrists.

Fun fact: A "Jennifer" tattoo used to be on Marc's right wrist. Let freedom ring.