Plant growth and survival depends to a large extent on the diurnal regulation of cellular processes. Although extensively studied at the transcript level, notably less is known about diurnal fluctuations at the protein level. Here, we report a high-resolution quantitative time-course of the Arabidopsis rosette proteome and phosphoproteome over a 12 h light:12 h dark diel cycle. We monitored the proteome every 2 h and the phosphoproteome immediately before and after the light-to-dark and dark-to-light transitions. Notably, we quantified nearly 5000 proteins and 1800 phosphopeptides, of which 288 and 225, respectively, were found to fluctuate over the time-course. Diurnal proteome and phosphoproteome changes were related to diverse biological processes, including protein translation, light detection, photosynthesis, metabolism and transport. Together, these datasets represent the most comprehensive proteomic analysis of Arabidopsis rosettes to date, allowing us to make multi-level inferences about the diurnal regulation of key cellular plant processes plants.