Aleyn (fl. c. 1400) was an English composer. Two of his works survive in the Old Hall Manuscript, one a Gloria (no. 8), the other a Sarum Agnus Dei discant (no. 3, Old Hall, no. 128), later scratched out, which is ascribed to W. Aleyn. If this inscription is correct, the conflation of this composer and Johannes Alanus, who wrote Sub Arturo Plebs, is incorrect. David Fallows ascribed this Agnus to W. Typp, however it seems that he was mistaken.
Aleyn is a surname. Notable people with the surname include:
- Simon Aleyn, the likely subject of the famous ballad, "The Vicar of Bray"
- Charles Aleyn
- William Aleyn
- John Aleyn (disambiguation)