The maximum shear modulus (G) is a key material characteristic that is incorporated in advanced soil constitutive models. Numerous experimental studies have been conducted to describe the effects of particle sizes and packing characteristics on G. However, most of these studies were conducted on quartz-based sands. A review of the literature revealed that few studies have described the effects of grain size distribution (GSD) on G in calcareous sands. Therefore, bender element (BE) tests were performed on calcareous sands with different mean grain sizes (d), uniformity coefficients (C), and void ratios to obtain G. The BE results revealed that the G of calcareous sand increases slightly with increasing d but decreases significantly with increasing C. A modified model of G incorporating the effects C and d was therefore developed for calcareous sand. Moreover, microscopic observations of pore size distributions (PSD) obtained from nuclear magnetic resonance (NMR) tests were presented to demonstrate the effect of GSD on PSD and its correlation with G. The NMR results revealed that the interaggregate pore structure proportion and uniformity of the PSD decreased significantly with increasing C but increased slightly with increasing d. The underlying mechanism for the effect of GSD on G was related to its substantial impact on microstructure. The significant decrease in G with increasing C can be attributed to the remarkable reduction in the ratio of the interaggregate void ratio to the intraaggregate void ratio. Additionally, G was enhanced as the heterogeneity of the microporosity structure distribution decreased. Graphic abstract: [Figure not available: see fulltext.].