Plants allocate a considerable amount of carbon (C) acquired through photosynthesis to fine root exudation, which in turn promotes microbial activity and contributes to soil heterotrophic respiration. However, few studies have evaluated the seasonality of root exudation in temperate forests, and the coordination between root exudation and root morphology and chemistry throughout the seasons remains unclear. In this study, we measured total C exudation from roots of three diameter classes (<0.5, 0.5–1.0, and 1.0–2.0 mm) for oak (Quercus serrata) and cedar (Cryptomeria japonica) throughout the year (February, April, May, July, August, September, and December) using filter paper exudation traps in a warm-temperate secondary forest in Japan. We also measured root morphological traits (specific root length [SRL]) and chemical traits (root nitrogen [N] content), as well as seasonal changes in the relationships between root exudation rates and these root traits. The generalized additive mixed model revealed peaks in root exudation during spring and summer, with no interspecific differences in root exudation rates. Seasonal variation in root exudation was more pronounced for roots with small diameters (<0.5 mm) than those with larger diameters (0.5–2 mm). The slope of the relationship between root exudation rates (y-axis) and SRL (x-axis) was steeper in summer (August) for oak, and in spring (April) and summer (August and September) for Cedar. Root exudation was also positively correlated with root N tissue concentration across all seasons, but the relationship remained consistent throughout the year. The maximum peak of root exudation in summer was likely due to the high photosynthetic activity during this season for both species studied. The peak observed in March and April, followed by declines in May and June, may reflect a shift in carbon allocation toward aboveground tissues during the leaf-expansion season in May and June. Also, root exudation from roots with high SRL in spring and/or summer would contribute to the peaks of seasonal variation in root exudation. We suggest that root exudation exhibits seasonal variation regardless of tree species, with seasonality being partially driven by coordination with root morphology rather than root chemistry.