<b><i>Background and Objectives:</i></b> Stem cell therapy is a novel treatment with regenerative ability that can treat erectile dysfunction (ED). This phase 1/2 clinical trial (NCT02945449) using 2 consecutive intracavernous (IC) injections of allogeneic Wharton’s jelly-derived mesenchymal stem cells (WJ-MSCs) was studied for the first time in the treatment of diabetic patients with ED. The primary outcome was to assess the safety and tolerability, and the secondary outcome was to assess the efficacy of 2 consecutive IC injections of allogeneic WJ-MSCs in diabetic ED. <b><i>Patients and Methods:</i></b> Twenty-two diabetic patients with refractory ED were included. Two consecutive IC injections of allogeneic WJ-MSCs were performed. Tolerability was assessed immediately, and at 24 h, safety was evaluated for 12 months. Efficacy was assessed using International Index of Erectile Function-5 (IIEF-5), Erection Hardness Score (EHS), and Color Duplex Doppler Ultrasound for 12 months. <b><i>Results:</i></b> The procedure was well-tolerated. Minimal and transient adverse events were redness and bruising at the site of injections. There were no patient-reported serious adverse effects. There were significant improvements in IIEF-5, EHS, peak systolic velocity (PSV) basal, and 20-min PSV, all over the follow-up time points in comparison to the baseline. <b><i>Conclusion:</i></b> This is the first human study with proven tolerability, safety, and efficacy of IC injections of allogeneic WJ-MSCs for the treatment of diabetic patients with ED.