Plant basic helix-loop-helix (bHLH) transcription factors constitute one of the largest and most functionally diverse transcription factor families in plants. Through a conserved bHLH domain, they bind E-box cis-elements or dimerize with partner proteins, coupling developmental and metabolic programs to environmental cues. However, a systematic synthesis of their multilayered stress responses remains lacking. Here, we review the structural features, genome-wide distribution, and evolutionary diversification of plant bHLH transcription factors, with emphasis on their regulatory functions under low-temperature, drought, salinity, and iron-deficiency conditions. We further integrate their roles in abscisic acid, jasmonic acid, and light signaling, as well as in anthocyanin and terpenoid biosynthesis, biotic stress, and immune responses. We also discuss emerging evidence linking bHLH factors to reproductive development, particularly anther and pollen formation under heat stress. Mechanistically, we summarize the multilayered regulatory network from four perspectives: transcriptional cascades, post-translational modifications, cooperative regulation by multiple transcription factors, and growth-defense trade-offs. Finally, we highlight current gaps, including limited functional validation in non-model/specialty economic crop species, underexplored combined-stress mechanisms, and lagging breeding translation, and propose future research directions. This review provides an integrated framework for plant stress biology and outlines evidence-based perspectives for future stress-tolerant crop improvement.