The integration of nanomaterials into cementitious composites has opened new avenues for improving mechanical and durability performance. Bacterial Nanocellulose (BNC), a sustainable, bio-derived nanomaterial, offers unique structural, chemical, and physical properties that can be harnessed to enhance concrete performance. However, comprehensive insights into its multifunctional behaviour in ordinary Portland cement (OPC) systems remain limited. This study investigates the effects of incorporating varying dosages (0.1�0.5 by weight of cement) of Bacterial Nanocellulose into M40 grade concrete, with a focus on optimizing fresh, mechanical, microstructural, and durability properties, including thermal and acid resistance. Nine concrete mixes (M0�M9) were prepared with incremental BNC dosages, and Mix M0 was designated as the control. Fresh properties were evaluated using slump and compaction tests. Compressive, split tensile, and flexural strengths were measured at 7, 28, and 52 days. Durability assessments included 90-day water absorption, water permeability, and acid resistance (in 5 sulfuric acid). Thermal resistance was evaluated by exposing specimens to 300 °C, 600 °C, and 900 °C. Microstructural characterization was conducted via SEM, EDAX, and FTIR spectroscopy. Results show that the workability of concrete decreased with increasing BNC content due to water entrapment by its hydrophilic nano-network. Mix M5 (0.3 BNC) exhibited optimal performance, with compressive, split tensile, and flexural strengths increased by 28.9 , 32.1 , and 29.2 , respectively, at 52 days. Water absorption and permeability in M5 decreased by 18.8 and 24.1 , respectively. Acid resistance improved by 62.5 in terms of weight loss compared to the control. Thermal resistance was also enhanced, with M5 retaining 35.0 of its original strength at 900 °C, compared to 30.5 in M0. Rebound hammer testing further confirmed an increase of 17.6 in surface hardness for M5, indicating enhanced near-surface density and mechanical integrity. FTIR analysis of Mix M5 revealed broadened Si�O and O�Si�O bands between 470.6 and 778.22 cm^�1, and a new peak at 1040.52 cm^�1 indicating C�O�C linkages from BNC glucopyranose units. Additional peaks at 1565.13 cm^�1 and 1749.32 cm^�1 confirmed the presence of carboxylic and lactone groups, while the suppression of carbonation-related peaks at 2346.24 cm^�1 and 2381.92 cm^�1 demonstrated reduced permeability and improved durability. BNC incorporation at 0.3 by weight of cement substantially improves the mechanical strength, pore structure refinement, moisture resistance, thermal stability, and acid durability of OPC-based concrete. These results highlight BNC as a promising multifunctional nano-additive for sustainable and durable infrastructure in aggressive service environments. © 2025 Elsevier B.V., All rights reserved.