Sulfur polymer mortars are composite materials in which modified sulfur is the binder while sand and other fillers are used as aggregates. Sulfur-based composite materials are characterized by their high chemical and mechanical properties along with fast setting behavior. However, sulfur modification is essential in order to achieve sulfur mortars with enhanced characteristics. In this study, incorporation of crumb tire rubber in dicyclopentadiene-modified sulfur-based mortars, as some replacement of sand, is evaluated. Use of crumb tire rubber influences the compressive stress-strain behavior of the mortars significantly. While the compressive strength of the rubber-added mortars is reduced dramatically, the postpeak response of the mortars is of the great interest. Use of a reactive-mixing method improves the compressive strength of rubber-added sulfur polymer mortars up to approximately 25%. Scanning electron microscopy observations confirm better adhesion between the rubber particles and sulfur polymer matrix via reactive mixing. Furthermore, it is clarified that the crumb tire rubber type and presence of a curing agent during the mortar preparation process affect the mechanical properties of the sulfur mortar.