Detrusor underactivity (DU) is diagnosed using urodynamic testing. We hypothesized that nocturia is associated with detrusor underactivity. We performed a retrospective chart review of all women who underwent urodynamic testing at our institution between 2016 and 2018. Uroflowmetry and pressure-flow study parameters were compared between women with nocturia (≥2 voids/night) and without nocturia (0-1 void/night). Detrusor underactivity was diagnosed using 3 different criteria: (1) bladder voiding efficiency (BVE) of <90%, (2) bladder contractility index of <100, and (3) a composite of three urodynamic measures (Gammie criteria). Of 358 women, 172 (48%) were in the nocturia group and 186 (52%) were in the no nocturia group. On uroflowmetry, median postvoid residual volume was similar (20 mL) in both groups. Median maximum flow rate (15 vs 17 mL/s, P < 0.05) and average flow rate (6 mL/s vs 7 mL/s, P < 0.05) were significantly lower in the nocturia group compared with the no nocturia group. During pressure-flow study, a significantly greater proportion of women with nocturia were unable to void around the catheter (30% vs 27%, P < 0.01). The overall rate of DU varied with the criteria used: BVE (54%), bladder contractility index (41%), and Gammie criteria (7%). The rate of DU using the BVE criteria was significantly higher in the nocturia group (63% vs 48%, P < 0.01), but no significant differences were noted using the other criteria. Nocturia is associated with reduced voiding efficiency in women. The diagnosis of DU using urodynamics is challenging.