Test results of three large-scale reinforced concrete (RC) deep beam specimens with a shear span-to-depth ratio (a/h) of 1.6 are reported in this paper. One control beam was not strengthened whereas two beams were strengthened in shear using one layer of unidirectional carbon fabric-reinforced matrix (CFRM) composites. A geopolymeric matrix was used in the CFRM of one of the strengthened specimens to examine its potential use as a sustainable alternative to a commercial cementitious mortar. The geopolymeric matrix was a mixture of ground granulated blast furnace slag and fly ash activated by an alkaline solution consisting of sodium silicate and sodium hydroxide. The control beam failed shortly after initiation of a diagonal splitting shear crack in the shear span. The strengthened beams failed at a higher load in a shear-compression or diagonal tension mode of failure. Shear strengthening with CFRM composites resulted in a shear strength gain of 95% when a cementitious mortar was used as a matrix. The use of CFRM with a geopolymeric matrix was effective in improving the shear response but to a lesser extent. The gain in shear capacity caused by CFRM with a geopolymeric matrix was 77%.