Background Posthumous data use policy within the broader scope of navigating post-mortem data privacy is a procedurally complex landscape for those engaged in data collection and sharing, and data-driven health research. Objective To help address some of this complexity by exploring patterns in individuals’ willingness to donate data with health researchers after death and developing practical recommendations. Methods An electronic survey was conducted in April 2021 among adults (>=18 years of age) registered in ResearchMatch (www.researchmatch.org), a national health research registry. Descriptive and multinomial logistic regression analyses were conducted at a 95% confidence level to determine the association between willingness to donate data after death overall and across each demographic category (education level, age range, duration of using online medical websites, annual frequency of getting ill) and based on the overall quantity at which individuals expressed willingness to donate data (some, all, none) with researchers. Results Of 399 responses, most participants were willing to donate health data (electronic medical record data [67%], prescription history data [63%], genetic data [54%], and fitness tracker data [53%]) after death. We identified that individuals were more likely to donate some data after death (versus no data) if they had longer duration of using online medical websites (relative risk ratio = 1.22, p<0.05, 95% CI: 1.01 to 1.48). No additional significant findings were observed between willingness to donate all, some, or none of their data after death and other demographic factors. Conclusions There are opportunities to enhance practices to engage patients in health systems’ online medical websites as one potential mechanism to encourage or inspire individuals in posthumous data donation for health research purposes.