Optically Stimulated Luminescence (OSL) dating of ancient mortars from a Roman monument of Braga (NW Portugal) is proposed in this work, using the single aliquot regenerative dose (SAR) protocol to estimate the equivalent dose on quartz grains of the mortar aggregate. The OSL signals of the quartz grains of the mortars indicate reliable signals for dating. The obtained age results fall within the Roman period (considering the error range) but there are considerable differences among samples. As OSL dating is based on changes induced in minerals by ionizing radiation, further research on the estimation dose rate of ionizing radiation of materials surrounding the samples and on the heterogeneity of the beta dose are needed to calculate more accurately the dose received by the grains and achieve a more accurate age.