Objectives: This study aims to assess the effect of ultralow dose 0.005% estriol vaginal gel in women with genitourinary syndrome of menopause (GSM). Methods: In this prospective and multicenter single-arm study, efficacy was assessed by the evaluation of the epithelial maturation value (MV), vaginal pH, symptoms and signs of vulvovaginal atrophy. Tolerability, acceptability, and the effect on intimate relationships were also evaluated. Results: We included 35 postmenopausal women with moderate-to-severe vaginal dryness. The most bothering symptom reported was vaginal dryness. The mean increase in the MV after 7 and 14 days of treatment were 22.1 (P < 0.001) and 39.9 (P < 0.001) points, with an increase in the superficial cells of 17.7 percentage points (pp) (95% confidence interval [CI], 7.9–27.4; P < 0.001) and 41.4 pp (95% CI, 28.2–54.6; P < 0.001) observed at the timepoints. Additionally, the pH decreased by 0.6 ± 0.7 (mean ± SD) at 7 days (P < 0.0001) and by 1.1 ± 0.8 at 14 days (P < 0.0001) from a baseline mean value of 6.3 ± 0.8. The severity of vaginal dryness (range, 0 [none] to 3 [severe]) was significantly reduced by a mean of 1.4 points (P < 0.0001) at 7 days and 2 points (P < 0.0001) at 14 days. Conclusions Ultralow dose 0.005% estriol vaginal gel produced a rapid improvement of most relevant symptoms and signs of GSM. This clinically meaningful response was observed from the initial days of treatment, confirming a fast onset and a progressive action.