Talk, Understand and Listen for InPatient Settings (TULIPS) was a multi-centred randomized control trial of an intervention that aimed to increase patient access to psychological therapies on acute mental health wards. This paper aims to: (i) describe a strategy for designing a psychological intervention that is implementable in inpatient mental health settings; (ii) describe methods for assessing the fidelity of interventions within these settings; (iii) report on the extent to which fidelity was achieved in the TULIPS trial. The TULIPS intervention was designed using information from a systematic review, stakeholder interviews, pilot work and a consensus workshop. We assessed fidelity to the model in terms of the delivery and dose of essential elements of the intervention, quality of intervention delivery, engagement of participants with the intervention and differentiation between the intervention and usual care. Although the TULIPS intervention targeted known barriers to the delivery of psychological interventions on mental health wards, we found issues in implementing aspects of the intervention that were dependent upon the participation of members of the multidisciplinary team. Psychologists were able to overcome barriers to delivering individual therapy to patients as this provision was not reliant on the availability of other staff. The intervention period in the study was 6 months. A greater period of time with a critical mass of psychological practitioners is needed to embed psychological interventions on inpatient wards. Our fidelity framework and assessment methods can be used by other researchers implementing and testing psychological therapies within inpatient environments.