Purpose This prospective study aimed to evaluate the difference between 99m Tc-PSMA single-photon emission computed tomography (SPECT)/CT and multiparametric magnetic resonance imaging (mpMRI) in the detection of primary prostate cancer (PCa). Materials and methods Fifty-six men with suspected PCa between October 2019 and November 2022 were prospectively enrolled in this study. The median age of the patients was 70 years (range, 29-87 years). Patients were divided into high-(Gleason score>7, n=31), medium- (Gleason score=7, n=6) and low-risk groups (Gleason score < 7, n=6). All patients underwent 99m Tc-PSMA SPECT/CT and mpMRI at an average interval of 3 days (range, 1-7 days). The maximum standardized uptake value (SUV max ), the minimum apparent diffusion coefficient (ADC min ), and their ratio (SUV max /ADC min ) were used as imaging parameters to distinguish benign from malignant prostatic lesions. Results Of the 56 patients, 12 were pathologically diagnosed with a benign disease, and 44 were diagnosed with PCa. 99m Tc-PSMA SPECT/CT and mpMRI showed no significant difference in the detection of primary PCa (kappa =0.401, P =0.002), with sensitivities of 97.7% (43/44) and 90.9% (40/44), specificities of 75.0% (9/12) and 75.0% (9/12), and AUC of 97.4% and 95.1%, respectively. The AUC of SUV max /ADC min was better than those of SUV max or ADC min alone. When SUV max /ADC min in the prostatic lesion was >7.0×10 3 , the lesion was more likely to be malignant. When SUV max /ADC min in the prostatic lesion is >27.0×10 3 , the PCa patient may have lymph node and bone metastases. SUV max was positively correlated with the Gleason score ( r =0.61, P=0.008), whereas ADC min was negatively correlated with the Gleason score ( r =-0.35, P =0.023). SUV max /ADC min was positively correlated with the Gleason score ( r =0.59, P =0.023). SUV max /ADC min was the main predictor of the high-risk group, with an optimal cut-off value of 15.0×10 3 . Conclusions The combination of 99m Tc-PSMA SPECT/CT and mpMRI can improve the diagnostic efficacy for PCa compared with either modality alone; SUV max /ADC min is a valuable differential diagnostic imaging parameter.