Background Human papillomavirus (HPV) is a common sexually transmitted infection, causing multiple cancers, including cervical, penile, and anal. Infection and subsequent health risks caused by HPV can be diminished by HPV vaccination. Unfortunately, vaccination rates among Hmong Americans are substantially lower than those among other racial and ethnic groups, despite having higher cervical cancer rates than non-Hispanic White women. Such disparities and sparse literature highlight the need for innovative and culturally appropriate educational interventions to improve HPV vaccine rates in Hmong Americans. Objective We aimed to develop and evaluate the effectiveness and usability of an innovative web-based eHealth educational website, the Hmong Promoting Vaccines website (HmongHPV website), for Hmong-American parents and adolescents to improve their knowledge, self-efficacy, and decision-making capacities to obtain HPV vaccinations. Methods Through social cognitive theory and community-based participatory action research process, we created a theory-driven and culturally and linguistically appropriate website for Hmong parents and adolescents. We conducted a pre-post intervention pilot study to assess the website’s effectiveness and usability. Overall, 30 Hmong-American parent and adolescent dyads responded to questions about HPV and HPV vaccine knowledge, self-efficacy, and decision-making at preintervention, 1 week after intervention, and at the 5-week follow-up. Participants responded to survey questions about website content and processes at 1 and 5 weeks, and a subset of 20 dyad participants participated in telephone interviews 6 weeks later. We used paired t tests (2-tailed) to measure the change in knowledge, self-efficacy, and decision-making processes, and used template analysis to identify a priori themes for website usability. Results Participants’ HPV and HPV vaccine knowledge improved significantly from pre- to postintervention stage and follow-up. Knowledge scores increased from preintervention to 1 week after intervention for both parents (HPV knowledge, P=.01; vaccine knowledge, P=.01) and children (HPV knowledge, P=.01; vaccine knowledge, P<.001), which were sustained at the 5-week follow-up. Parents’ average self-efficacy score increased from 21.6 at baseline to 23.9 (P=.007) at post intervention and 23.5 (P=.054) at follow-up. Similar improvements were observed in the teenagers’ self-efficacy scores (from 30.3 at baseline to 35.6, P=.009, at post intervention and 35.9, P=.006, at follow-up). Collaborative decision-making between parents and adolescents improved immediately after using the website (P=.002) and at follow-up (P=.02). The interview data also revealed that the website’s content was informative and engaging; in particular, participants enjoyed the web-based quizzes and vaccine reminders. Conclusions This theory-driven, community-based participatory action research–designed and culturally and linguistically appropriate educational website was well received. It improved Hmong parents’ and adolescents’ knowledge, self-efficacy, and decision-making processes regarding HPV vaccination. Future studies should examine the website’s impact on HPV vaccine uptake and its potential for broader use across various settings (eg, clinics and schools).