Investigating alternative protein sources for ruminants in tropical regions is crucial due to distinct climatic conditions and limited resources. Scientists, acknowledging the challenges, are exploring locally accessible alternatives to develop cost-effective and sustainable protein supplements for ruminant nutrition, emphasizing the importance of understanding the nutritional composition for advancing livestock husbandry sustainability. The aim of this research is to elucidate alternative local protein sources for ruminants in tropical areas and assess their effectiveness in promoting sustainable production and enhancing ruminant nutrition. Protein feed sources are essential for providing amino acids to animals and producing animal products such as meat, milk, and wool. Inadequate protein in animal feed may have a detrimental impact on rumen fermentation and animal output. Therefore, alternative protein sources have received increased attention in order to replace the main protein source. The alternative protein sources in this review were divided into five categories: agricultural biomass, fodder shrubs, biological protein and plant weeds, insect protein and agro-industry biomass. Furthermore, plant secondary compounds found in alternative protein sources might be used to modulate rumen fermentation and improve animal performance. By focusing on regional sources of protein, feed costs are reduced while availability is raised. Based on this analysis, it can be concluded that there are a number of alternative protein sources that might be used in place of primary protein sources and that they could support ruminant production in the face of several animal feed crises. However, the potential use of any protein source should be assessed in light of the country’s availability and animal production practices.