In recent years, interest has been growing in the use of geopolymer lightweight concrete in construction industry to capitalize on its advantages of low cost and ecological impact. This study evaluates the effects of basalt fiber on some mechanical properties and high-temperature performance of lightweight geopolymer concrete. For this purpose, a solution of sodium hydroxide and sodium silicate was used as alkali activator to activate ground granulated blast furnace slag and metakaolin. Concretes containing 0.2% basalt fiber yielded the highest strength compared to all other concretes. Concretes containing 0.2% basalt fiber yielded 14%, 27.6%, and 142.7% higher compressive strength, flexural strength, and modulus of elasticity, respectively, compared to the reference concrete. Regardless of basalt fiber content, the compressive strengths of all lightweight geopolymer concretes indicated a decreasing trend up to 600°C, but after this temperature, the compressive strengths of all concretes increased significantly. At 700°C, the compressive strengths of the geopolymer concretes ranged from approximately 26.8% to 45.5% compared to their pre-exposure strengths. Scanning electron microscopy and energy dispersive spectroscopy analyses revealed a more consistent structure at the paste-pumice interface than at the paste-limestone interface. Because the physical structures of geopolymer concrete mixtures containing fibers are quite similar, X-ray diffraction analysis indicated that no significant difference was observed between the peaks formed due to an increase in basalt fiber content. Due to the enhancing effect of basalt fiber on the mechanical properties of lightweight geopolymer concretes, the performance of lightweight geopolymer concretes after exposure to high temperatures was superior to that of Portland cement lightweight concretes. As a result, the usability of structural lightweight geopolymer concrete in construction industry is remarkable, especially in terms of sustainability in terms of high temperature performance.