The use of steel slag, as an alternative to natural aggregate, contributes to sustainable development. Potential dimensional instability of the slag aggregate is a concern for use in construction, especially in hot climates. This paper investigates the potential use of steel slag aggregate to replace imported aggregate in hot mix asphalt in Qatar. Laboratory expansion tests and a weathering trial were conducted to assess the long-term expansion properties of the slag. Asphalt mixtures were developed in the laboratory with steel slag aggregate of different sizes and content, and selected mixtures were trialled in a full-scale road section. Field performance was evaluated immediately after construction and after three years in service. Whilst the slag mineralogy indicated the presence of unhydrated lime, the laboratory test and weathering trial confirmed low susceptibility to expansive reactions. Increasing the slag content resulted in higher optimum bitumen content. Coarse slag aggregate was more effective than fine slag aggregate in achieving the Marshall Mix design requirements. The field performance showed improved resistance of the slag asphalt to deformation, with no adverse effect on moisture susceptibility or bitumen ageing compared to the control mix.