TRIANGLE is a multicentre trial investigating whether the addition of a novel intervention for patients and carers (ECHOMANTRA) to treatment as usual (TAU) improves outcomes for people with anorexia nervosa (AN). ECHOMANTRA is based on the cognitive interpersonal model of AN and includes assessments, workbooks, videos, online groups and joint Skype sessions for patients and carers. People receiving intensive hospital treatment (N = 380) will be randomised to TAU or TAU plus ECHOMANTRA. Participants will be assessed over an 18‐month period following randomisation. The primary outcome is patient psychological well‐being at 12 months postrandomisation. Secondary outcomes include (i) patient's weight, eating disorder symptoms, motivation to change, quality of life and number of days in hospital at 12 months postrandomisation and (ii) carer's psychological well‐being, burden and skills at 12 months (some outcomes will be assessed at 18 months postrandomisation). The results from this trial will establish the effectiveness of ECHOMANTRA. Trial registration: ISRCTN registry ISRCTN14644379, 08/12/2016. Copyright © 2017 John Wiley & Sons, Ltd and Eating Disorders Association.