BACKGROUND Spousal bereavement is both common in later life and often a major adverse life event. Despite frequently affecting their mental health, older adults, particularly in Portugal, encounter barriers in seeking mental health support during bereavement. This is mainly due to a scarcity of grief-specific support, and a tendency to dismiss the struggle as ‘natural’ and undeserving of specific support by the bereaved and other healthcare professionals. OBJECTIVE To evaluate the feasibility, acceptability, effectiveness and costs of the web-based self-help program (EuLuto), designed to address grief and mental health challenges in older adults experiencing spousal bereavement. METHODS In this before and after pilot study we assigned 30 Portuguese-speaking participants aged 55+ and experiencing spousal bereavement to use EuLuto, an web-based, asynchronous, self-help, and psychoeducational program for spousal bereavement for 10 weeks. The primary outcome was grief symptoms (Texas Revised Inventory of Grief (TRIG)), secondary outcomes were prolonged grief (PG-13), depression, and loneliness. Additionally, costs were estimated and a qualitative study on barriers and facilitators using a sub-sample of participants was conducted. RESULTS Prolonged grief (PG-13) and depression scores, but not grief (TRIG) and loneliness, showed a significant decrease after 10 weeks of intervention. The qualitative study demonstrated that main barriers and facilitators to the use of EuLuto were the motivation of users and psychological capability. CONCLUSIONS Despite preliminary positive effects and overall acceptability of EuLuto, due to limitations in design and sample size, the study could not establish effectiveness at this time. This paves the way for a randomized controlled trial with a larger sample. CLINICALTRIAL https://clinicaltrials.gov/study/NCT05156346 INTERNATIONAL REGISTERED REPORT RR2-10.2196/37827