The polyphenolic composition of Camellia irrawadiensis, which is a closely related species of Camellia sinensis (cultivated tea), was investigated. The most predominant polyphenol, a kind of ellagitannin, was isolated from leaves of C. irrawadiensis. Its structure was established as 1,2-di-O-galloyl-4,6-O-(S)-hexahydroxydiphenoyl-β-D-glucose (2) on the basis of spectral and chemical evidence. Moreover, the polyphenols [catechins, strictinin (1), compound 2, theogallin, and gallic acid] and two methylxanthines (theobromine and caffeine) in leaves of C. irrawadiensis were determined by HPLC-Photodiode array detector analysis, and were compared to those in C. sinensis and Camellia taliensis. Total catechin content in C. irrawadiensis was lower than that in C. sinensis and C. taliensis. Theobromine content in C. irrawadiensis was higher than that in C. sinensis and C. taliensis. The content of 2 in C. irrawadiensis was 8.4% of dry leaf weight and comprised approximately 60% of the total polyphenols detected, while the compound was not detected in C. sinensis and was reported to be 2.4% in C. taliensis.