Abstract: Budget execution remains a major challenge in Kenya’s devolved governance system, with many county governments failing to implement approved budgets effectively. Budget reliability, defined as the extent to which actual revenues and expenditures align with approved budget estimates, is considered essential for effective budget execution. This study examined the effect of budget reliability on budget execution of county governments in Kenya. Guided by the Theory of Fiscal Decentralization, the study adopted an explanatory research design under a positivist research philosophy. Data was collected using structured questionnaires from 332 respondents drawn from finance, budget, planning, and audit departments in Nairobi City, Kiambu, Machakos, and Kajiado counties. A response rate of 85.52% was achieved. Secondary data were obtained from Office of the Controller of Budget reports. Data was analyzed using simple linear regression analysis in SPSS version 27. The findings revealed that budget reliability has a positive and statistically significant effect on budget execution (β = 0.701, t = 3.143, p = 0.004). The regression model explained 48.6% of the variation in budget execution (Adjusted R² = 0.486), while the ANOVA results confirmed that the model was statistically significant (F = 74.112, p < 0.05). The study concludes that counties with credible and predictable budgets are better positioned to improve service delivery, fiscal discipline, and accountability. The study recommends strengthening revenue forecasting, ensuring timely disbursement of funds, improving expenditure monitoring, managing pending bills effectively, and diversifying own-source revenue to enhance budget reliability and budget execution performance.