The World Health Organization (WHO) has recently made recommendations for treating all Human Immunodeficency Virus (HIV/AIDS) patients irrespective of their immune status. This issue poses challenges to lower-middle-income countries like Nigeria due to resource limitations. This study performed a budget impact analysis (BIA) of different strategies for the initiation of antiretroviral therapy (ART) among individuals living with HIV/AIDS in Nigeria. A dynamic cohort budget impact model was used to compare two initiation arms (CD4 < 350 cells/ml and CD4 > 500 cells/ml), with CD4 > 500 cells/ml representing the 2015 WHO guidelines for initiation of ART. Outcomes were hospitalization costs, new infection transmission, and overall budget impact. Key inputs included HIV prevalence in Nigeria, ART access and costs, hospitalization rates and costs, ART uptake trends, and transmission rates. Sensitivity analysis empolyed Monte Carlo simulations to assess the impact of selected parameters. At the end of year five, applying the 2015 WHO guidelines reduced new HIV transmissions by 87.0%, preventing 77,000 infections. Hospitalization cost reductions saved $1.12 million. Overall budget impacts were $718 million for immediate initiation versus $903 million for deferred initiation, yielding $184 million in savings. Monte Carlo simulations showed reduced transmission as the main driver of savings. Prioritizing early treatment initiation espoused by the 2015 WHO guidelines maximize resource efficiency, reduces long-term healthcare costs, and accelerate progress toward epidemic control targets. The findings strongly support the adoption and sustained implementation of the 2015 WHO guidelines for immediate ART initiation. It has public health and economic benefits.