In this study, behaviour of the waste steel fibre reinforced alkali-activated slag mortars exposed to high temperatures is investigated. For this purpose, mortar mixtures are produced by activating blast furnace slag with sodium silicate (Na2SiO3) and sodium hydroxide (NaOH). In addition to the mixtures produced without waste fibre, the mortar mixtures including 1% and 2% waste fibre by volume are manufactured and the influence of waste steel fibre content on the mechanical properties of the alkali activated mortars is investigated. The specimens with the dimensions of 4x4x16 cm taken from the mortar mixtures are exposed to 200 degrees C, 400 degrees C and 600 degrees C temperatures and the changes due to the elevated temperature effect on the unit weights, ultrasound pulse velocities, flexural and compressive strengths of the mortars are determined experimentally. Results show that the waste steel fibres improve the mechanical properties of the alkali-activated mortars. Besides, the flexural and compressive strengths of the alkali activated mortars are increased in different proportions after exposure to 200 degrees C and 400 degrees C temperatures.