Ground granulated blast furnace slag (GGBFS) is an industrial by-product material that its application in ground improvement projects is a well-established practice. This study is focused on investigating the effect of GGBFS on drained shear strength characteristics of an over-consolidated clay by performing a series of direct shear tests. The tests are conducted on untreated clay and clay treated with 3%, 5%, and 7% GGBFS (by dry weight of the soil). The results depicted that increasing the GGBFS contents increased the mobilised peak and residual shear strength of the tested clays. The cohesion values in fully softened and residual conditions showed an increasing trend upon increasing the GGBFS contents and curing time. However, in the residual condition, it dropped to a very low range (i.e., 2.8 ≤ cr ≤ 8 and 2.8 ≤ cr ≤ 13.6 kPa). Further investigation showed that fully softened and residual friction angle (ϕĨƐ, ϕƌ) and failure envelope slope line (mfs, mr) slightly decreased by increasing the GGBFS content and curing time.