Cindy Lou Who is a character featured in Dr. Seuss's storybook How the Grinch Stole Christmas! and its adaptations. She is a two-year-old Who girl who lives in Whoville and plays a pivotal role in the story.

In the original book and the animated adaptation, Cindy Lou Who gets out of bed to go get a drink of water when she finds The Grinch, disguised as Santa Claus, stuffing her family's Christmas tree up the chimney. Mistaking him for Santa, she innocently asks him why he's taking the tree. The Grinch, sensing innocence in the young girl, covers up his theft by fibbing to her, saying that there is a broken light on the tree, and that he is taking it to "his workshop" to fix the problem and then return it on Christmas Day when she wakes up. The Grinch then gets her drink and sends her back to bed, showing the only legitimate kindness he has in his tiny heart. Although this is her only real scene in the original book, Cindy Lou gets a bit more screen time in the animated adaptation, such as in the opening songs and at the end as the Grinch celebrates with the Whos.

In the 2000 film version, Cindy Lou is at best eight to ten years old and knows of the Grinch early on in the story thanks to her doing some investigative reporting (her father, never shown in the book or the TV special, is the Whoville Postmaster, or head mailman, who occasionally delivers the papers as well). Unlike the rest of the Whos, Cindy believes there is some good in the Grinch. Augustus May Who, the Mayor of Whoville, angrily accuses her of robbing everyone when the Whos awake to their homes being ransacked by the Grinch during the night in a bid to ruin Christmas for them. However, her name is cleared shortly thereafter, and she later helps the Grinch return every item back to the Whos after sneaking aboard his sleigh.