Importance Hydrocolloid dressings (HCDs) are designed to promote wound healing; however, there are no randomized clinical trials comparing changes in scar appearance using HCD vs daily petroleum ointment after excisional surgery. Objective To determine whether a 1-time application of HCD for 1 week after excisional surgery affects scar appearance and surgical complications compared to daily petroleum ointment. Design, Setting, and Participants This was an investigator-blinded, individually randomized clinical trial conducted from October 2022 to October 2023 at a large public university in Indiana of adult (≥18 years) patients undergoing standard excision or Mohs micrographic surgery with linear bilayered repair. Exclusion criteria included flap or graft use, topical chemotherapy agents used on surgical site, allergy to adhesives, scar in hair-bearing site, previous use of HCD, and communication barriers. Data analysis was conducted from November 2023 to March 2025. Interventions HCD application over the excisional surgery site for 1 week or daily reapplication of petroleum ointment. Main Outcomes and Measures Patient-reported scar appearance using a modified Visual Analog Scale (VAS). Secondary outcomes included VAS scores from 3 Mohs surgeons who were blinded, rates of surgical complications, and patient ratings for comfort and convenience. Results Of the 444 screened, 146 patients (mean [SD] age, 61.9 [12.9] years; 85 female [58%] and 61 male [41.8%] individuals) were randomized, 72 to HCD and 74 to petroleum ointment; demographic characteristics were similar between groups. Surgeon and patient ratings for overall appearance were clinically comparable. At 7 days, the difference in mean VAS rating between HCD and petroleum groups was −0.40 (95% CI, −0.70 to −0.10); at 30 days, it was −0.08 (95% CI, −0.38 to 0.23); and at 90 days, −0.09 (95% CI, −0.41 to 0.23). The HCD group had higher, but not statistically significant, rates of adverse events, including postoperative bleeding (20.6% for HCD vs 8.8% for petroleum), wound dehiscence (6.2% vs 0), and surgical site pain (21.2% vs 12.3). No patients required postoperative antibiotics. A greater proportion in the HCD group rated the HCD as convenient or extremely convenient (86.9% vs 46.8%; difference, 40.1% [95% CI, 24.9% to 55.3%]) and comfortable or extremely comfortable (73.8% vs 48.3%; difference, 25.4% [95% CI, 8.7% to 42.2]). Conclusions and Relevance This randomized clinical trial found that HCD is a suitable postoperative option after dermatologic surgery, yielding similar scar appearance and complication rates as daily petroleum ointment. Therefore, the clinical decision to use HCD vs daily petroleum ointment should balance cost and patient preferences with their risk of postoperative complications. Trial Registration ClinicalTrials.gov Identifier: NCT05618912