Background Harnessing supportive influences in close relationships is an innovative and potentially effective strategy to improve sun protection behaviors. Objective This pilot randomized controlled clinical trial evaluates the feasibility and impact of Sun Safe Partners Online, a web-based, couples-focused intervention to improve sun protection behavior. Methods A total of 75 couples reporting suboptimal levels of sun protection recruited from Facebook advertisements were randomized to receive a web-based intervention called Sun Safe Partners Online or a Generic Online Sun Safety Information intervention. Sun Safe Partners Online had 4 individual-focused modules and 4 couples-focused modules. Feasibility was assessed by study enrollment, engagement, follow-up survey completion, and intervention evaluation. Participants completed baseline and a 1-month postintervention survey assessing sun protection and exposure, along with individual and relationship attitudes about the importance of sun protection. Results Using Facebook as a recruitment strategy resulted in rapid enrollment and higher acceptance than for the prior telephone and print trial. The follow-up survey completion was higher in the Generic Online condition (100%) than in the Sun Safe Partners Online condition (87.2%). Engagement in Sun Safe Partners Online was high, with more than two-thirds of participants completing all modules. Evaluations of Sun Safe Partners Online content and features as well as ease of navigation were excellent. Sun Safe Partners Online showed small effects on sun protection behaviors and sun exposure on weekends compared with the Generic Online intervention and moderate effect size increases in the Sun Safe Partners Online condition. Conclusions This study uses a novel approach to facilitate engagement in sun protection by harnessing the influence of relationships among spouses and cohabiting partners. A couples-focused intervention may hold promise as a means to improve sun protection behaviors beyond interventions focused solely on individuals by leveraging the concern, collaboration, and support among intimate partners and addressing relationship-based barriers to sun protection. Trial Registration ClinicalTrials.gov NCT04549675; https://clinicaltrials.gov/ct2/show/NCT04549675