Purpose To perform an external validation of a recently published nomogram aimed to predict positive 68 Ga-PSMA-11 PET/CT in patients with biochemical recurrence (BCR) after radical prostatectomy (RP) by Rauscher et al. (Eur Urol 73(5):656–661, 2018 ). Methods Overall, 413 PCa patients with BCR after RP (two consecutive PSA ≥ 0.2 ng/ml) and PSA value between 0.2 and 1 ng/ml were included. A multivariable logistic regression model was produced to assess the predictors of positive 68 Ga-PSMA-11 PET/CT results. The performance characteristics of the model were assessed by quantifying the predictive accuracy, according to model calibration. Yuden’s index was used to find the best nomogram’s cut-off. Finally, decision curve analysis (DCA) was implemented to quantify the nomogram’s clinical value. Results In the external cohort, the overall detection rate of 68 Ga-PSMA-11 PET/CT was 44% vs. 64.7% in the original population. At multivariate analysis, PSA at 68 Ga-PSMA-11 PET/CT (OR: 7.06, p  < 0.001) and ongoing ADT at time of 68 Ga-PSMA-11 PET/CT (OR: 2.07, p  = 0.03) were the only independent predictors of PET/CT positivity. The predictive accuracy of nomogram was suboptimal and comparable to that reported in the original model (64% vs. 67%, respectively). The calibration plot indicated suboptimal concordance. The best nomogram’s cut-off to predict positive 68 Ga-PSMA-11 PET/CT was 35% (AUC = 0.61). In DCA, the nomogram revealed clinical net benefit when the threshold probabilities of positive 68 Ga-PSMA-11 PET/CT is > 35%. Conclusion We assessed similar suboptimal predictive accuracies in the external cohort compared to the original one. PSA and ongoing ADT were confirmed as positive predictors, and the most informative nomogram cut-off resulted 35%.