Belief systems comprising religious affiliation, levels of religiosity, and cultural traditions are essential in shaping individual experiences of grief. They guide social practices and psychosocial responses. This study explored the influence of belief systems on spousal grief among widowed individuals in Siaya County, Kenya. The selected county has the highest national proportion of widowed persons. The study was guided by Social Constructionism Theory. A convergent mixed-method approach and a descriptive design were adopted to investigate the interplay between belief systems and spousal grief. Data were collected through questionnaires, in-depth interviews, and focus group discussions. A sample size of 399 respondents was derived from an estimated population of 114,216 widowed individuals in Siaya County using Cochran formula, and selected through multi-stage cluster sampling. On the other hand, key informants were purposively sampled. Quantitative data were analysed using frequency counts, percentage counts, mean, and standard deviation. Inferentially, regression was used to establish the relationship between belief systems and spousal grief. Thematic analysis was used for qualitative data. Compliance with ethical considerations was ensured throughout the study. From the findings, it emerged that belief systems were both supportive and had negative impacts on spousal grief (p < 0.001; R² = 0.20). Having strong religious affiliation and high levels of religiosity were linked with psychosocial consolation and community support for widowed individuals. The study recommends that the Ministry of Public Service, Gender and Affirmative Action should upscale targeted interventions to challenge and transform adverse socio-cultural practices associated with widowhood in Siaya County.