The Pinocchio paradox arises when Pinocchio says "My nose grows now" and is a version of the liar paradox. The liar paradox is defined in philosophy and logic as the statement "This sentence is false." Any attempts to assign a classical binary truth value to this statement lead to a contradiction, or paradox. This occurs because if the statement "This sentence is false" is true, then it is false; this would mean that it is technically true, but also that it is false, and so on without end. Although the Pinocchio paradox belongs to the liar paradox tradition, it is a special case because it has no semantic predicates, as for example "My sentence is false" does.

The Pinocchio paradox has nothing to do with Pinocchio being a known liar. If Pinocchio were to say "I am getting tired," this could be either true or false, but Pinocchio's sentence "My nose grows now" can be neither true nor false; hence this and only this sentence creates the Pinocchio (liar) paradox.

Pinocchio is a hero of the 1883 children's novel The Adventures of Pinocchio by Italian author Carlo Collodi. Pinocchio, an animated puppet, is punished for each lie that he tells by undergoing further growth of his nose. There are no restrictions on the length of Pinocchio's nose. It grows as he tells lies and at one point grows so long that he can not even get his nose "through the door of the room".

The Pinocchio paradox was proposed in February 2001 by 11-year-old Veronique Eldridge-Smith. Veronique is the daughter of Peter Eldridge-Smith, who specializes in logic and the philosophy of logic. Peter Eldridge-Smith explained the liar paradox to Veronique and Veronique's older brother and asked the children to come up with their own versions of the famous paradox. In a few minutes, Veronique suggested: "Pinocchio says, 'My nose will be growing'." Eldridge-Smith liked the formulation of the paradox suggested by his daughter and wrote an article on the subject. The article was published in the journal Analysis, and the Pinocchio paradox became popularized on the Internet.

The paradox suggested by Veronique, "My nose grows now", or in future tense: "will be growing", leaves room for different interpretations. In the novel, Pinocchio's nose continues to grow as he lies: "As he spoke, his nose, long though it was, became at least two inches longer." So logicians question if the sentence "My nose will be growing" was the only sentence that Pinocchio spoke, did he tell a lie before he said "My nose will be growing", or was he going to tell a lie—and how long would it take for his nose to start growing?

The present tense of the same sentence "My nose is growing now" or "My nose grows", appears to provide a better opportunity to generate the liar paradox.

The sentence "My nose grows" could be either true or false.

Assume the sentence: "My nose grows now" is true: