Sofia Vergara Flaunts Bare Butt While Tanning on Boat, Video

Sofia Vergara's keeping her clothing casual while enjoying a chill Fourth of July weekend ... letting everyone see her impressive backside while tanning.

The actress shared a vid from her vacation in Ibiza in honor International Bikini Day on Saturday ... in which she's lying on her front in a thong-style bikini -- meaning no coverage at all on her booty.

Like we said, she's lying on her front so we can't see her bikini top -- but, the strings in back don't seem to be tied ... so, it seems safe to assume she's letting it all hang out.

Vergara gives the person shooting a wave and a big grin ... not at all concerned about how much skin she's showing.

Sofia's no stranger to European vacations obviously ... just recently going to France with Heidi Klum -- her "America's Got Talent" co-judge who is also know for stripping down.