The main purpose of this study was to examine the moment, ductility and toughness capacities of steel and hybrid (steel + synthetic) fiber-reinforced concrete (FRC)-filled square aluminum (AL), carbon steel (CS) and stainless steel (SS) tube beams subjected to four-point-in-plane bending. A total of 9 hollow, 18 plain and 72 steel and hybrid FRC-filled AL, CS, and SS beams were tested under four-pointbending until failure. The effects of the steel tube thickness (2, 3 and 4 mm), fiber type (steel and hybrid), fiber volume ratio (0.5% and 1.5%), and the compressive strength of concrete (30 and 70 MPa) on the moment, ductility and flexural toughness capacity of low- and high-strength steel and hybrid FRC-filled AL, CS and SS beams were examined. The results showed that while the steel and hybrid fibers considerably increased the ductility and toughness capacities of the AL, CS and SS beams, they did not significantly contribute to their moment capacities. When compared to plain concrete filled AL, CS and SS beams, although the greatest increase in the moment capacities of the 1.5% steel and hybrid FRC-filled AL, CS and SS beams were only 7.65%, 2.88%, 2.28% and 9.12%, 3.68%, 12.1%, respectively for 70 MPa compressive strength of concrete, the increase in ductility capacities of these beams were 26.6%, 64.3%, 95.2% and 29.9%, 85.9%, 98.9%, respectively. Furthermore, the increase in pre-peak and post-peak energy absorption capacities of 1.5% steel and hybrid FRC-filled AL CS, and SS beams were obtained as 7.86%, 3.26%, 6.30% and 26.57%, 38.38%, 53.44%, respectively for 70 MPa compressive strength of concrete. (C) 2019 Elsevier Ltd. All rights reserved.