Background: Web-based lifestyle interventions to promote healthy diet and physical activity among cancer survivors and their partners are recent developments; therefore, few studies have reported patterns of website use or associations with behavior change. Objective: The primary aim was to describe website use in the DUET (Daughters, Dudes, Mothers, and Others Together) trial and examine the associations between website use and changes in diet quality, moderate to vigorous physical activity (MVPA), and body weight. Methods: This secondary analysis used data from 28 survivor-partner dyads (BMI ≥25 kg/m2) randomized to the 6-month DUET web-based weight loss intervention, which released weekly e-learning sessions on diet and exercise. Website use was quantified as weeks of access, time spent, and frequency of page views. Diet quality was assessed using 2-day dietary recalls; MVPA was measured by the Godin Leisure-Time Exercise Questionnaire and accelerometry. Weight was measured on a scale. Website use was summarized descriptively, and associations were examined using Spearman partial correlations. Results: Participants had a mean age of 58 (SD 12.5) years; 78.6% (44/56) identified as female, 66.1% (37/56) were non-Hispanic White, and 86% (24/28) were breast cancer survivors. On average, participants viewed 11.2 (SD 7.4) weeks of the 24-week intervention, or a total of 312.9 (SD 255.7) minutes per participant. Sessions (n=2736), Home Page (n=975), and Tools (n=967) features showed the highest activity (5885 total page views). Website use was higher among adults aged 65 years and older than younger participants, showcased by duration of use (mean 14.4, SD 7.4 weeks vs mean 9.2, SD 6.8 weeks; P=.009), time spent per week (mean 17.0, SD 9.7 minutes vs mean 10.5, SD 10.6 minutes; P=.01), and total number of page views (mean 135.7, SD 90 vs mean 85.3, SD 111.9; P=.008); higher website use was also reported among women versus men in terms of duration of use (mean 12.8, SD 7.1 weeks vs mean 5.6, SD 5.9 weeks; P=.003), time spent per week (mean 14.6, SD 10.3 minutes vs mean 7.4, SD 10.3 minutes; P=.02), and total number of page views (mean 120, SD 110.2 vs mean 50.3, SD 64.4; P=.01). Diet quality was positively associated with website use (weeks: r=0.50; P<.001; time: r=0.45; P<.001; total page views: r=0.46; P<.001; and sessions page views: r=0.39; P=.005). Self-reported MVPA was also positively associated with website use (weeks r=0.37; P=.007; time: r=0.36; P=.009; total page views: r=0.36; P=.01; and sessions page views: r=0.35; P=.01). No significant associations were detected for accelerometry-measured MVPA or weight. Conclusions: Cancer survivors and their partners engaged with the DUET web-based platform to support diet and physical activity (with use particularly high among older adults and females). However, larger, more diverse dyadic web-based lifestyle interventions are needed to confirm these findings. Clinical Trial: ClinicalTrials.gov NCT04132219; https://clinicaltrials.gov/study/NCT04132219
