The Three Gorges Reservoir (TGR) in China is the largest hydroelectric project in the world, but the threat of sediment affecting ecological sustainability of the reservoir is a topic of concern. Sediment particle-size distribution (PSD) is informative in understanding sediment transport dynamics and biochemical functions. It is, therefore, important to quantitatively characterize the distribution of sediment particles. In the current study, fractal theory is applied to determine the PSD of suspended sediment in the TGR. The results show that the volumetric fractal dimension $\left(D_{\mathrm{v}}\right)$ exhibits a significant seasonal difference (p < 0.05), reflecting sediment source and hydrodynamic sorting control the granularity of suspended sediment in the TGR. More specifically, suspended sediment particles are coarser in the wet season than in the dry season for the Yangtze River, and the opposite is true for the Ruxi River, an important tributary. The generalized dimension spectrum, $D\left(q\right)–q$, and multifractal singularity spectrum, $f\left[\alpha \left(q\right)\right]–\alpha \left(q\right)$, were calculated for each suspended sediment sample. Thereafter, the parameters, D(0), D(1), D(2), α(0), Δα(q), and $\Delta f\left[\alpha \left(q\right)\right]$, were determined to characterize the PSD. As a result, the coarser suspended sediment during the wet season is characterised by a more complex PSD pattern, with a wider range of particle sizes, greater heterogeneity, and greater homogeneity of distribution over the measurement interval. However, the multifractal structure of the PSD of suspended sediment is more complex during the dry season than during the wet season, with higher local dispersion and variability. The findings of the current study highlight that multifractal analysis provides important insight for understanding the PSD of suspended sediment in the TGR.