<p>Naturally occurring radionuclides are applied as tracers to characterize sediment in aquatic systems. Sediment samples were collected from various sites within the Mkomazi River Basin, South Africa and analysed by means of gamma spectroscopy. The concentrations of the three naturally occurring radionuclides (⁴⁰K, ²³⁸U and ²³²Th) were combined to derive an arbitrary activity, which was found to have a strong correlation with the average grain size of the samples. A mathematical relation was extracted to estimate the average sediment grain size by using this arbitrary activity, and a mean estimation error of 17.2% was achieved. Findings in this study area suggest that ²³⁸U concentrations are the most significant predictor of sediment grain size, while ²³²Th has minimal influence. This study highlights the potential for in situ radionuclide measurements to estimate sediment grain size in aquatic environments, offering a novel approach for sediment transport studies.</p>