Soil-geosynthetic interface shear strength is an essential parameter for the design and stability analysis of geosynthetic-reinforced soil structures. Economic and environmental reasons have led to increasing use of locally available residual soils with a significant percentage of fines and lower draining capacity, when compared with the traditional good-quality backfill materials. This paper describes an extensive laboratory study carried out using a large-scale direct shear test device, in which the influence of soil moisture content, soil density and geosynthetic type on the direct shear behaviour of the soil-geosynthetic interface was evaluated. The study involved a locally available granite residual soil and four geosynthetics: two geogrids (one uniaxial and the other biaxial), one geocomposite reinforcement (high-strength geotextile) and one geotextile. Test results have revealed that the increase in soil moisture content can measurably reduce the soil-geosynthetic interface shear strength. Regardless of soil moisture content, soil density proved to have a remarkable influence on interface shear strength, particularly when geogrids were used. Among the different geosynthetics tested, the biaxial geogrid was found to be the most effective reinforcement for this particular type of soil, concerning the direct shear mechanism. For soil-geogrid interfaces, the coefficients of interaction ranged from 0.71 to 0.99. For soil-geotextile interfaces, the coefficients of interaction varied from 0.54 to 0.85.