A triplet-triplet annihilation-based photon upconversion (TTA-UC) system with a low threshold excitation intensity (Ith) in plain water was developed. Water-soluble anionic porphyrin (PdTPPS4-) and diphenylanthracene (DCDPA2-) derivatives were used as light absorbers and emitter molecules, respectively, and no additives such as surfactants were required. The phosphorescence emission from PdTPPS4- under an excitation wavelength of 528 nm was quenched by DCDPA2-, resulting in triplet energy transfer, whereas fluorescence from DCDPA2- was observed in a short wavelength region (400-500 nm). Three independent emission studies utilizing different excitation light sources validated the TTA-UC process in a simple aqueous solution. TTA occurred after the triplet energy transfer, according to the time profiles of phosphorescence and fluorescence detected following pulse laser excitation. The Ith for TTA-UC was estimated to be lower than 6 mW cm-2, although it could not be exactly determined due to the sensitivity limit of the experimental setup. The upper limit of Ith for the aqueous solution of DCDPA2- and PdTPPS4- is the smallest value obtained to date for aqueous systems and comparable to that of high-performance TTA-UC systems in organic solutions.