The 39-year-old "Wild" star shared the picture of her pet with fans on Instagram, reported E! online.
"Drum roll please! Introducing the newest member to our family #LouAnn !! #bullterrier #madlyinlove (just look at that face *sigh*)," she captioned the picture.
Witherspoon made her followers play a mystery game, by providing riddles before announcing the latest addition.
"What's white and black and fuzzy and cute and sweet and charming? Any guesses?," she quizzed.
To give fans some hint, the "Legally Blonde" actress shared the picture of LouAnn's ear.
