Introduction: Iron deficiency anemia (IDA) and β-thalassemia trait (BTT) are the two most frequent causes of microcytic anemia; however, differentiation between these conditions is of clinical importance for treatment purposes. The aim of this study was to determine the reliability of red cell distribution width (RDW) versus red cell distribution width index (RDWI) as hematological discriminator indices to differentiate between IDA and BTT in an adult Omani population. Methods: This cross-sectional study was conducted in 2019 at various primary health care centers in Muscat Governorate, Oman. A retrospective review was conducted of adult Omanis participating in the national premarital screening program. Identification of IDA and BTT was performed according to the diagnostic criteria of the World Health Organization. Subsequently, the validity of RDW and RDWI as discriminator indices to differentiate the two conditions were evaluated according to their sensitivity, specificity, positive predictive value, negative predictive value, and Youden’s index (YI). In addition, a receiver-operating characteristic (ROC) curve analysis was performed to calculate the area under the curve (AUC). Results: A total of 156 cases (15.5%) of IDA and 113 cases (11.3%) of BTT were identified among 1,004 individuals undergoing premarital screening. Neither of the two discriminator indices showed sensitivity and specificity rates of 100% and both had low YI values of <0.5. However, the AUC for RDWI was 0.81 compared to 0.62 for RDW. Conclusions: RDWI appears to be a good discriminator index to differentiate IDA from BTT.