The phytochrome photoreceptors regulate plant growth and development throughout their life cycle. Rice (Oryza sativa) possesses three phytochromes, phyA, phyB, and phyC. Physiological, genetic, and biochemical analyses of null mutants of each phytochrome have revealed the function of each in rice. However, few studies have investigated the relationship between phytochrome signaling and metabolism. In the present study, non-targeted metabolite analysis by gas chromatography time-of-flight mass spectrometry (GC/TOF-MS) and targeted metabolite analysis by capillary electrophoresis electrospray ionization mass spectrometry (CE/ESI-MS) were employed to investigate metabolic changes in rice phyA phyB phyC triple mutants. Distinct metabolic profiles between phyA phyB phyC triple mutants and the wild type (WT), as well as those between young and mature leaf blades, could be clearly observed by principal component analysis (PCA). The metabolite profiles indicated high accumulation of amino acids, organic acids, sugars, sugar phosphates, and nucleotides in the leaf blades of phyA phyB phyC triple mutants, especially in the young leaves, compared with those in the WT. Remarkable overaccumulation of monosaccharide, such as glucose (53.4-fold), fructose (42.5-fold), and galactose (24.5-fold), was observed in young leaves of phyA phyB phyC triple mutants. These metabolic phenotypes suggest that sugar metabolism, carbon partitioning, sugar transport, or some combination of these is impaired in the phyA phyB phyC triple mutants, and conversely, that phytochromes have crucial roles in sugar metabolism.