Viscosity-enhancing admixtures (VEAs) are required in the production of self-consolidating concrete (SCC). This paper presents the rheological properties in rotational and oscillatory shear tests of cement pastes and mortars with a w/c ratio of 0.50, containing nopal mucilage and marine brown algae extract, which are proposed as two new VEAs. For comparison purposes, a commercial VEA based on welan gum was used. In addition, preliminary SCC mixtures were prepared using the proposed and the commercial VEA. The slump flow, J-ring, L-box, V-funnel and static segregation column were used to evaluate the SCC fresh state properties. The results indicated that the new VEAs produced a significant increase on the shear viscosity and the yield-stress of pastes, mortars and SCC, which increased with increasing concentration and depending on their molecular nature. This was more noticeable in the mixes containing brown algae extract, which may be related to chains’ entanglement and gel-network formation. Cement pastes and mortars containing the proposed VEAs exhibited a solid-like behavior at short values of strain amplitude (<1%). The proposed VEAs produced some stable and homogeneous SCC mixtures in comparison with the commercial VEA based in welan gum.