UNLABELLED: Limited involvement in treatment-related decision making can affect adolescent and young adult (AYA) cancer outcomes and well-being. Information on developmentally consistent approaches to enhance involvement in and experiences with decision making is lacking. In a pilot randomized trial, we evaluated the feasibility, acceptability, and usability of a Web-based decision support intervention (DECIDES) for treatment-related decisions, with/without coach support. Newly diagnosed/relapsed AYA (15-24 y old) and caregivers were randomized to usual care ( = 11), DECIDES ( = 21), or DECIDES+ ( = 21 with a coach). Feedback on DECIDES was obtained in interviews with AYA and caregivers (DECIDES, DECIDES+) and oncology clinicians ( = 13). Feasibility, acceptability, and usability are described. Quantitative data were integrated with qualitative data. Mean differences (MDs) between DECIDES groups and usual care at 8 wk after randomization are presented for knowledge, decision-making involvement, and decision processes. High retention was achieved. High acceptability and good-excellent usability of DECIDES were reported; qualitative data were congruent with these ratings. AYA and caregivers with a coach demonstrated higher engagement with DECIDES. Based on the MDs, as compared with usual care, AYA (DECIDES and DECIDES+) reported higher shared decision making (MD = 12.58, 11.93), higher decision-making involvement (MD = 19.31, 9.78), and lower decision regret (MD = -13.19, -16.55), respectively, and caregivers (DECIDES, DECIDES+) reported higher shared decision making and AYA decision-making involvement. Minimal changes to knowledge were observed. DECIDES is feasible, acceptable, and usable for AYA treatment-related decision making with possible increases in AYA involvement. Findings suggest that decision support interventions warrant further evaluation for AYA across the treatment trajectory and for a range of treatment-related decisions. Delivery closer to the initial diagnosis and inclusion of a coach for engagement may be advantageous. This study is registered at ClinicalTrials.gov (NCT ID No. NCT06191679). HIGHLIGHTS: Adolescents and young adults with newly diagnosed cancer, and their caregivers, endorsed that decision support is important for their understanding of cancer and treatment and for enhanced communication with their health care teams.Implementation of a decision support intervention shortly after cancer diagnosis is acceptable, feasible, and usable for adolescents and young adults and their caregivers.Oncology clinicians confirm the usability of decision support interventions for their adolescent and young adult patients at diagnosis.Access to a decision support intervention at diagnosis may improve the involvement of adolescents and young adults in their treatment-related decision making and decision processes.Future studies can continue to refine the content and delivery of decision support interventions close to the time of diagnosis, including value of a coach and the role of the caregiver, and targeted to a range of treatment-related decisions.