Importance: Other than nasal moisturizers, no standard-of-care medical therapy exists for epistaxis in hereditary hemorrhagic telangiectasia (HHT). With epistaxis as the greatest cause of morbidity in patients with HHT, there is a need to identify effective topical therapies. Objective: To determine the efficacy and safety of an intranasal timolol thermosensitive gel vs placebo thermosensitive gel in treating HHT-associated epistaxis. Design, Setting, and Participants: This double-blind, placebo-controlled randomized clinical trial was conducted from October 29, 2019, to May 20, 2020, at a tertiary care center. A total of 27 patients with HHT and moderate-to-severe epistaxis were recruited and included in this prespecified analysis: 14 in the timolol group and 13 in the placebo group. Inclusion criteria included (1) age 18 years or older, (2) clinical or genetic diagnosis of HHT, (3) screening Epistaxis Severity Score (ESS) of 4 or greater and 2 or more nosebleeds cumulatively lasting at least 5 minutes per week, (4) stable epistaxis pattern over the preceding 3 months, and (5) no change in epistaxis treatment or nasal hygiene regimen in the preceding month. Exclusion criteria included (1) contraindications to systemic β-blocker administration, (2) use of medications interacting with timolol, (3) use of antiangiogenic medications in the last month before recruitment, and (4) use of anticoagulants, antiplatelets, or fibrinolytic therapies within the last month. Interventions: Novel thermosensitive intranasal timolol (0.1%) gel vs placebo thermosensitive gel applied twice daily to each nostril for 8 weeks. Main Outcomes and Measures: The primary outcome was the median change in ESS and percentage of participants reaching the minimal clinically important difference in ESS. Secondary outcomes were changes in Clinical Global Impression-Severity and Clinical Global Impression-Improvement scores, Nasal Outcome Score for Epistaxis in Hereditary Hemorrhagic Telangiectasia, and hemoglobin level. Results: Of 27 participants randomized (median [range] age, 55 [20-76] years; 14 women [52%]; 25 White [93%]), a total of 23 patients with HHT completed the primary outcome measure. Within the timolol gel and placebo gel groups, respectively, the median change (range) in ESS was 2.32 (0.22 to 5.97) vs 1.96 (-0.91 to 5.98), and 9 of 11 (82%) vs 9 of 12 (75%) participants experienced a clinically meaningful improvement in ESS. Twenty-two of the 23 participants (96%) reported improvement via the Clinical Global Impression-Improvement score, with 81% vs 58% of participants reporting reduced severity of epistaxis in the timolol vs placebo group, respectively. Of participants completing the Nasal Outcome Score for Epistaxis in HHT at follow-up visit, 7 of 10 (70%) in the timolol group achieved a clinically important difference vs 5 of 10 (50%) in the placebo group. There was no change in hemoglobin level between or within groups. Zero participants in the placebo group and 2 of 13 (15%) in the timolol group withdrew because of adverse events. Conclusions and Relevance: Thermosensitive gel, alone or in combination with timolol, was highly effective in reducing HHT-associated epistaxis. The timolol group had greater improvement in epistaxis and quality of life than the placebo group, but effect estimates were imprecise, and no definitive conclusions on the superiority of timolol can be drawn. Physicians treating patients with HHT-associated epistaxis should consider a thermosensitive gel (with or without timolol) for their patients. Trial Registration: ClinicalTrials.gov Identifier: NCT04139018.