Provided are a user authentication device and method based on iris recognition. The user authentication method includes: acquiring an image of a user's left and right eyes; extracting a pupil image and an iris image from the image; obtaining a first iris feature by analyzing the iris image; obtaining a second iris feature by analyzing the iris image and the pupil image; obtaining a similarity score based on the first iris feature, the second iris feature, and prestored reference iris data; and determining whether to approve user authentication based on the similarity score.