This pilot trial aimed to evaluate feasibility and acceptability of co-designed unguided internet CBT for grief. Efficacy was assessed for anxiety (primary outcome), well-being, depression, posttraumatic stress disorder (PTSD), and prolonged grief (secondary outcomes), relative to four-week wait-list control. There were 88 participants, M age = 16.95 years, SD = 1.45, who had experienced the death of a person (n = 72; 81.82%) or other loss (e.g. parental divorce; n = 16; 18.81%). Participants were randomised to intervention (n = 49; 55.68%), or waitlist control (n = 39; 44.31%). Outcome measures included the Revised Children's Anxiety and Depression Scale, Prolonged Grief Disorder Scale Revised, Children's Revised Impact of Events Scale, and the WHO-5 Well-Being Index. Findings suggest good feasibility, with acceptable recruitment and attrition (n = 9; 18.37% of 49 intervention participants at post-intervention). While no differences in anxiety, depression, PTSD, or prolonged grief were observed between intervention and waitlist control post-intervention, there was a significant small between groups effect on well-being in favour of intervention. There was a significant within group reduction for intervention participants in anxiety, depression and PTSD at follow-up. Acceptability was high, 97% (n = 33 of 34 completers) liked the program, however uptake was moderate.Trial Registration: The trial was registered on 26 March 2024 with the Australian and New Zealand Clinical Trials Registry (ACTRN12624000337572).