Abstract: Priming—a state of heightened defense readiness induced in plants by prior exposure to mild stimuli before encountering pathogens or abiotic stresses—has emerged as a frontier strategy in recent years for synergistically enhancing both crop resistance and yield. The conventional view holds that biochar contributes to crop health primarily by improving soil physicochemical properties or indirectly promoting beneficial microbiota. This review proposes a new paradigm: when applied as a seed coating, biochar can be directly perceived by plants as a complex physical and chemical signal, thereby triggering systemic priming effects. This primed state enables plants to mount faster and stronger responses to subsequent biotic and abiotic stresses throughout their entire growth cycle. Here, we summarize recent advances in biochar seed coating–induced plant immunity and stress resistance. The potential mechanisms underlying biochar-induced priming include: 1) Physical signal perception: the rough edges and porous structure of biochar particles impose continuous mild mechanical stimulation on radicles, triggering calcium influx and reactive oxygen species (ROS) bursts that initiate defense pathways; 2) Chemical signal induction: signal molecules carried by biochar—such as polyphenols, lignin degradation products, karrikins, as well as released mineral ions and free radicals—act as endogenous elicitors or damage-associated molecular patterns (DAMPs) to stimulate plant immunity; 3) Microbe-mediated effects: biochar creates a unique niche around seeds, enriching beneficial microorganisms such as Pseudomonas, Bacillus, and actinomycetes, which elicit jasmonic acid/ethylene-dependent induced systemic resistance (ISR). Following biochar-induced priming, plants exhibit enhanced sensitivity to ROS, Ca²⁺, and nitric oxide (NO) signaling, leading to rapid activation of MAPK cascades. This results in coordinated regulation of salicylic acid, jasmonic acid, and ethylene pathways, upregulation of transcription factors such as WRKY and MYB, and increased expression of pathogenesis-related (PR) genes. Moreover, epigenetic reprogramming of defense-related genes and accumulation of secondary metabolites are reinforced. Case studies demonstrate that biochar seed coating significantly reduces the incidence of fungal and bacterial diseases—including tomato Fusarium wilt, strawberry gray mold, and bacterial wilt—while also conferring cross-tolerance under viral infections, drought, salinity, and heavy metal stress. Future research should focus on: 1) decoupling physical, chemical, and microbial effects through approaches such as size fractionation, chemical extraction, and sterilization treatments; 2) designing functionally targeted “immunity-enhancing” biochars via feedstock selection and pyrolysis parameter optimization; 3) elucidating the receptors and signaling pathways involved in plant recognition of biochar-derived signals; and 4) conducting long-term field trials and cost–benefit analyses to advance the industrial application of biochar seed coating technology.