BackgroundTherapist-delivered online cognitive–behavioural therapy (CBT) has been found to be effective for depression in primary care.AimsTo determine the cost-effectiveness of online CBT compared with usual care.MethodEconomic evaluation at 8 months alongside a randomised controlled trial. Cost to the National Health Service (NHS), personal costs, and the value of lost productivity, each compared with outcomes based on the Beck Depression Inventory and quality-adjusted life-years (QALYs). Incremental analysis indicated the NHS cost per QALY gain.ResultsOnline CBT was more expensive than usual care, although the outcomes for the CBT group were better. Cost per QALY gain based on complete case data was £17 173, and £10 083 when missing data were imputed.ConclusionsOnline CBT delivered by a therapist in real time is likely to be cost-effective compared with usual care if society is willing to pay at least £20 000 per QALY; it could be a useful alternative to face-to-face CBT.