BackgroundLoneliness is common among older adults in institutional settings. It leads to adverse effects on health and wellbeing, for which nature contact with peers in turn may have positive impact. However, the effects of nature engagement among older adults have not been studied in randomised controlled trials (RCT). The “Friends in Nature” (FIN) group intervention RCT for lonely older adults in Helsinki assisted living facilities (ALFs) aims to explore the effects of peer-related nature experiences on loneliness and health-related quality of life (HRQoL). In this study we aim describe the participants’ baseline characteristics of the RCT, feasibility of FIN intervention and intervention participants’ feedback on the FIN.MethodsLonely participants were recruited from 22 ALFs in Helsinki area, Finland, and randomised into two groups: 1) nature-based social intervention once a week for nine weeks (n = 162) and 2) usual care (n = 157). Demographics, diagnoses and medication use were retrieved from medical records, and baseline cognition, functioning, HRQoL, loneliness and psychological wellbeing were assessed. Primary trial outcomes will be participants’ loneliness (De Jong Giervald Loneliness Scale) and HRQoL (15D).ResultsThe mean age of participants was 83 years, 73% were female and mean Minimental State Examination of 21 points. The participants were living with multiple co-morbidities and/or disabilities. The intervention and control groups were comparable at baseline. The adherence with intervention was moderate, with a mean attendance of 6.8 out of the nine sessions. Of the participants, 14% refused, fell ill or were deceased, and therefore, participated three sessions or less. General subjective alleviation of loneliness was achieved in 57% of the intervention participants. Of the respondents, 96% would have recommended a respective group intervention to other older adults. Intervention participants appreciated their nature excursions and experiences.ConclusionsWe have successfully randomised 319 lonely residents in assisted living facilities into a trial about the effects of nature experiences in a group-format. The feedback from participants was favourable. The trial will provide important information about possibilities of alleviating loneliness with peer-related nature-based experiences in frail residents.Trial registrationClinicalTrials.gov, ID: NCT05507684. Registration 19/08/2022.