The NLTT catalog is a collection of stars with proper motions exceeding 0.18"/yr found by Luyten and his coworkers. The First Supplement to the NLTT catalog is the result of continued plate analysis and contains data for 398 stars with proper motions greater than 0.179"/yr. A number of corrections to the data in the published catalog have been entered in the machine-readable version, including the omission of duplicate entries and those stars later determined to show no motion. The catalog contains, in addition to the proper motions, 1950 equatorial coordinates, a flag indicating the approximate accuracy of the positions, estimated red and photographic magnitudes, spectral types, and flags indicating a change from the published volume and the presence of the star in the supplement.