Portland cement is extensively used as a binder in concrete production. However, with Portland cement production, 5% of the natural resources used in this production are consumed, constituting 5–7% of the total CO2 emission. In order to mitigate the environmental problems associated with cement production, styrene-butadiene rubber latex was used as cement replacement up to 20%. In this study, compressive strength, flexural strength, unit weight, water absorption, open porosity, water sorptivity and the chloride ion permeability of Portland cement mortar mixtures modified by styrene-butadiene rubber (SBR) polymeric latex were investigated. For this purpose, the sand/cement ratio and the water/cement ratio were kept constant as 3/1 and 0.5, respectively. In addition to the control mixture containing no polymer, 1, 2, 3, 5, 10 and 20 wt.% of cement was replaced with SBR. In this way, seven mortar mixtures were prepared. Mixed curing (wet cure and dry cure) method was applied to the mortar specimens. Results showed that up to a 5% replacement level, it is possible to improve the mechanical properties of cement mortars with SBR latex addition. However, at a 10% and 20% replacement level, SBR had a significant detrimental effect on the mechanical properties of polymer modified mortars. However, the transport properties decreased with the incorporation rate of SBR latex and the detrimental effect of SBR replacement was more pronounced in 20% SBR mortar mixtures.