Microplastic contamination in agricultural soil is an increasing environmental concern, yet its impact on the complicated interactions between plants and microbes within the rhizosphere remains insufficiently understood. While extensive research has shown the accumulation of microplastics in terrestrial ecosystems, fewer studies have revealed how microplastic-induced changes in rhizosphere microbial communities cause plant-specific responses to soil stress. This study investigated whether low-density polyethylene (LDPE) microplastics alter the rhizosphere microbial composition of Phaseolus vulgaris and whether these alterations explain cultivar-specific susceptibility or resilience. We hypothesized that LDPE contamination would disrupt microbial diversity, with more pronounced dysbiosis correlating with increased phenotypic stress in susceptible cultivars. Bean cultivars, pinto and kidney, were grown in soil with and without 2.5% (w/w) LDPE microplastics. Following a 14-day exposure period, morphological phenotypes were observed, and rhizosphere bacterial communities were characterized via 16S rRNA gene sequencing. Both cultivars exhibited root darkening and stunted growth under microplastic stress; however, kidney beans had more severe wilting and a more substantial reduction in height. The Shannon Diversity Index decreased by 12.9% in treated kidney beans, compared to a 6.2% reduction in pinto beans. Furthermore, microplastic exposure induced a sharp increase in the relative abundance of Proteobacteria and an overrepresentation of Pseudomonas in the kidney bean rhizosphere, whereas pinto beans maintained higher levels of beneficial taxa, such as Rhizobium. These findings demonstrate that LDPE microplastics significantly alter rhizosphere microbial dynamics in a cultivar-specific manner, suggesting that host–microbe interactions are pivotal in determining plant resilience to emerging soil contaminants.