Excessive consumption of concrete in rapid industrialization and urbanization harms global ecosystems by exhausting natural resources. Concurrently, turning industrial wastes into alternate construction materials provide protection of unusual mining of natural resources and promotes eco-friendly construction practices. Although some studies have spearheaded on the mechanical and durability aspects of concrete, incorporating recycled refractory brick aggregate, however, its behaviour in different structural components remains fully unexplored. The present article has enlightened the flexural behaviour of reinforced concrete beam with recycled refractory brick as an alternate of natural fine aggregate by casting four reinforced concrete beams having dimension of 1600 × 100 × 150 (L × B × H) mm. The tension and compression zones of the beam have been designed with two 12 mm and 6 mm dia bars respectively, considering under-reinforced section. The beams were tested under four-point bending method using a computerized servo-hydraulic 1000 kN loading frame. Three LVDTs and two strain gauges were attached to measure the deflection and strain values at different positions. Observations indicate that the Control Beam experiences its initial crack at a load of 22.9 kN, reaching an ultimate load of 60.4 kN with a centre deflection of 10.02 mm. In contrast, the Sustainable Beam exhibits its first crack at a load of 20.5 kN, achieving an ultimate load of 59.4  kN, and showcasing a centre deflection of 14.2 mm. The present study has highlighted the performance of recycled refractory brick as an alternative of natural fine aggregate on the flexural performance of the reinforced concrete beam with a limited scope.