Background Recent studies have reported the additive value of combined gallium 68 (Ga)-labeled Glu-urea-Lys (Ahx)-HBED-CC ligand targeting the prostate-specific membrane antigen (PSMA) (hereafter called Ga-PSMA-11) PET/MRI for the detection and localization of primary prostate cancer compared with multiparametric MRI. Purpose To compare the diagnostic accuracy and interrater agreement of multiparametric MRI and Ga-PSMA-11 PET/MRI for the detection of extracapsular extension (ECE) and seminal vesicle infiltration (SVI) in patients with prostate cancer. Materials and Methods Retrospective analysis of 40 consecutive men who underwent multiparametric MRI and Ga-PSMA-11 PET/MRI within 6 months for suspected prostate cancer followed by radical prostatectomy between April 2016 and July 2018. Four readers blinded to clinical and histopathologic findings rated the probability of ECE and SVI at multiparametric MRI and PET/MRI by using a five-point Likert-type scale. The prostatectomy specimen served as the reference standard. Accuracy was assessed with a multireader multicase analysis and by calculating reader-average areas under the receiver operating characteristics curve (AUCs), sensitivity, and specificity for ordinal and dichotomized data in a region-specific and patient-specific approach. Interrater agreement was assessed with the Fleiss multirater κ. Results For multiparametric MRI versus PET/MRI in ECE detection, respectively, AUC, sensitivity, and specificity in the region-specific analysis were 0.67 and 0.75 ( .07), 28% (21 of 76) and 47% (36 of 76) ( .09), and 94% (529 of 564) and 90% (509 of 564) ( .007). For the patient-specific analysis, AUC, sensitivity, and specificity were 0.66 and 0.73 ( .19), 46% (22 of 48) and 69% (33 of 48) ( .04), and 75% (84 of 112) and 67% (75 of 112) ( .19), respectively. For multiparametric MRI versus PET/MRI in SVI detection, respectively, AUC, sensitivity, and specificity of the region-specific analysis were 0.66 and 0.74 ( .21), 35% (seven of 20) and 50% (10 of 20) ( .25), and 98% (295 of 300) and 94% (282 of 300) ( < .001). For the patient-specific analysis, AUC, sensitivity, and specificity were 0.65 and 0.79 ( .25), 35% (seven of 20) and 55% (11 of 20) ( .20), and 98% (137 of 140) and 94% (131 of 140) ( .07), respectively. Interrater reliability for multiparametric MRI versus PET/MRI did not differ for ECE (κ, 0.46 vs 0.40; = .24) and SVI (κ, 0.23 vs 0.33; = .39). Conclusion Our results suggest that gallium 68 (Ga)-labeled Glu-urea-Lys (Ahx)-HBED-CC ligand targeting the prostate-specific membrane antigen (PSMA) (Ga-PSMA-11) PET/MRI and multiparametric MRI perform similarly for local staging of prostate cancer in patients with intermediate-to-high-risk prostate cancer. The increased sensitivity of Ga-PSMA-11 PET/MRI for the detection of extracapsular disease comes at the cost of a slightly reduced specificity. © RSNA, 2019.