WITH the exception of supernova outbursts, or changes in the pulsational periods of Cepheid variables arising from their changing internal structure, the evolution of stars is generally much too slow to have been detected during the era of modern astronomy. A few stars, such as the red supergiants ρ Cas and RW Cep, have shown spectral changes on a timescale of decades which have been attributed to evolutionary effects1, but these changes are mostly erratic and the evolutionary interpretation is uncertain. Here we report an analysis of modern and historical (back to about ad 1700) photometric measurements of the star P Cygni. We find a steady change of apparent brightness, and argue that it is due to evolution of the star. The change is about twice as fast as standard models predict, but the difference may be due to mis-estimation of the star's mass or inadequate treatment of atmospheric expansion in the stellar models.