In this study, the shear behavior of reactive powder concrete (RPC) in terms of shear strength and shear slip was studied by direct shear tests on cubic specimens and the influences of water-binder ratio, steel fiber volume ratio and normal compressive stress ratio were investigated. The test results indicated that using a low water-binder ratio and a reasonable fiber content could enhance the shear strength of RPC specimens. The RPC specimen containing steel fiber will not be severed into two individual parts due to the connection of steel fiber. Moreover, the normal stress was the key contribution to the shear strength, and the residual shear capacity was mainly provided by the friction of the shear surface. The cracking of the specimens became more severe but the shear strength in the cyclic horizontal test did not decrease obviously compared with that in the monotonic horizontal test. Applying the twin-shear unified strength theory to the test data, it was indicated that the elliptic curve showed a good agreement. Therefore, the unified twin shear strength theory could predict the proper RPC strength envelope curve under combined compressive-shear stress states.