Since the effect of aggregate fineness modulus on the mechanical performance and, in particular, the durability behavior of cement‑based materials containing recycled aggregates has rarely been studied to date, this research aimed to evaluate the role of this parameter on the performance of mortars made with natural and recycled aggregates of different fineness moduli. To this end, six mortar mixtures with a constant water‑to‑cement ratio of 0.6 were prepared using either entirely natural or entirely recycled aggregates. The fineness modulus of the aggregates was adjusted to three levels: 2.5, 3, and 3.5. A series of mechanical and durability tests were then conducted on the specimens, including compressive strength, volumetric water absorption, capillary water absorption, rapid chloride penetration and migration (RCPT/RCMT), and half‑cell potential measurement. The obtained results showed that in mortars containing recycled aggregates, increasing the fineness modulus led to improved compressive strength, reduced probability of corrosion, and enhanced durability properties, including lower volumetric water absorption, capillary water absorption, and chloride penetration/migration. In contrast, mortars made with natural aggregates exhibited an opposite trend: as the fineness modulus increased, compressive strength decreased and the probability of corrosion rose. Other durability indicators, such as volumetric water absorption, capillary water absorption, and chloride penetration/migration, were also adversely affected. For example, at 90 days of age, increasing the fineness modulus from 2.5 to 3.5 raised the compressive strength of recycled‑aggregate mortars by about 20%, whereas it decreased that of natural‑aggregate mortars by approximately 18%.