A novel ex-core-detector–based core power reconstruction method is presented. The method uses power correlations between fuel regions and can be applied to a real-time small reactor core monitoring system especially for the detection of abnormal behavior. The use of ex-core detectors reduces the installation and maintenance costs of small modular reactors (SMRs) compared to conventional in-core detectors. To construct the power distribution with ex-core-detector count rates, it is necessary to account for the scattering and absorption reactions of neutrons within the core that make it difficult to extract information directly from the central core region. In the proposed method, detector responses and power correlations are preevaluated and revised by mathematical transformation. Monte Carlo simulations using the realistic SMR core design MoveluXTM demonstrated that the present method is capable of reconstructing the core power distributions within an average error of 10% using the count rates of the ex-core detectors. Also, the reconstruction successfully identified the position of abnormal power peaks in the central core region and an unbalanced power distribution.