The thermoluminescence (TL) properties of feldspar were used to investigate the sediment movement processes in a fluvial–coastal system. Field samples were collected at various locations along the Tenryu River and the Enshunada Coast in Japan. After a series of pretests, an appropriate TL measuring sequence was proposed for this study. Applying this approach, the natural TL intensity of feldspar grains was measured. Owing to the young age of the research area, it was assumed that the natural TL difference was ascribed to the sample depositional environment under which different possibilities of sunlight exposure exist. Surface sediment particles in the target area were classified into three groups in terms of TL intensities, i.e., river sand with large TL signals, coastal sand with medium TL signals, and dune sand with small TL signals. Stratified configuration of the Nakatajima Coastal Dune was observed from the underground-sample TL glow curves with a top, windblown, dune-sand layer; a bottom, wave-induced, coastal sand layer; and a mixing layer in between. A rather complex sediment-movement pattern in front of the Tenryu River mouth was revealed after investigation of the underwater samples. Because of the seawater influence, acting as an ultraviolet filter to sunlight, underwater samples present a larger, high-temperature TL peak than the low-temperature peak; whereas these two are almost the same for ground samples.