Charlize Theron Heats Up Mexican Beach Ahead of New Year's Eve

Charlize Theron is heating things up down in Mexico ahead of the New Year ... enjoying some fun in the sun in a jaw-dropping black swimsuit.

Check it out ... the actress looked practically ageless as she donned a strapless one-piece bathing suit while abroad with her family south of the border Saturday.

She tried to keep a low profile, choosing to wear her platinum blonde locks -- featuring brunette peekaboo highlights -- up in a messy bun. Theron wore classic, black cat-eye shades and a beige fedora ... which perfectly rounded out her beach bombshell aesthetic.

Charlize went down to the water with one of her daughters to get a feel for the sand and surf ... skipping rocks across the calm blue sea.

It's rare time off for the perpetually busy actress-producer ... who has quite a few movies on the way -- including a recently revealed part in Christopher Nolan's next flick.

If you don't know ... Nolan is making a film adaptation of Homer's "The Odyssey" -- the classic Greek mythology tale about Odysseus, who spent 10 years trying to return to Greece after the Trojan War.

Theron was among the first wave of casting announcements ... along with Zendaya, Matt Damon, Tom Holland, Anne Hathaway, Robert Pattinson and Lupita Nyong'o.