Aims The aim of our study was to investigate noninvasive predictors for detrusor underactivity (DUA) in male patients with lower urinary tract symptoms (LUTS) and benign prostatic enlargement (BPE). Methods A consecutive series of patients aged 45 years or older with non‐neurogenic LUTS were prospectively enrolled. Patients underwent standard diagnostic assessment including International Prostatic Symptoms Score, uroflowmetry, urodynamic studies (cystometry and pressure‐flow studies), transrectal ultrasound of the prostate, and ultrasound measurements of the bladder wall thickness (BWT). Logistic regression analysis was used to investigate predictors of DUA, defined as a bladder contractility index < 100 mm H2O. A nomogram was developed based on the multivariable logistic regression model. Results Overall 448 patients with a mean age of 66 ± 11 years were enrolled. In a multivariable logistic age‐adjusted regression model BWT (odds ratio OR: 0.50 per mm; 95% confidence interval CI, 0.30‐0‐66; P = .001) and Qmax (OR: 0.75 per mL/s; 95% CI, 0.70‐0.81; P = .001) were significant predictors for DUA. The nomogram based on the model presented good discrimination (area under the curve AUC: 0.82), good calibration (Hosmer‐Lemeshow test, P > .05) and a net benefit in the range of probabilities between 10% and 80%. Conclusions According to our results, BWT and Qmax can noninvasively predict the presence of DUA in patients with LUTS and BPE. Although our study should be confirmed in a larger prospective cohort, we present the first available nomogram for the prediction of DUA in patients with LUTS.