J.Lo & Marc Anthony -- REUNITED in Red Convertible

Jennifer Lopez and Marc Anthony's separation has taken a back seat.

Nearly a month after the couple split, the two singers were spotted basking in the Long Island sun near Sagg Main Beach in Southampton ... inside a red Mustang convertible.

No explanation yet for the unexpected reunion today -- but Marc's rep tells TMZ, he was on the East Coast visiting his children.

5:32 PM: A rep for J.Lo tells TMZ, "They are NOT back together -- they were just visiting each other and the children."