Tomato cultivation faces significant challenges from foliar fungal diseases such as early blight caused by Alternaria spp., resulting in substantial reduction in yield. In this study, we examined the suppressive effects of the antagonistic fungus Clonostachys rosea on early blight of tomato under controlled experimental conditions. The results of dual culture experiments revealed the inhibitory effects of five C. rosea isolates on Alternaria spp., with the TNAU CR04 isolate exhibiting the highest inhibition (77.22%). Scanning electron microscopy provided good insight into antagonistic effects of C. rosea against A. alternata, revealing hyphal interactions and structural alterations. Further investigations focused on the suppression of Alternaria alternata mycelial growth (86.78%) by culture filtrates of C. rosea. The results revealed that TNAU CR04 at a 50% concentration strongly inhibited mycelial growth. Through GC-MS analysis, we identified key compounds involved in the interaction between C. rosea TNAU CR04 and A. alternata, shedding light on metabolic pathways and defense mechanisms. Overall, this study showed that C. rosea and its metabolites strongly act against Alternaria spp., revealing its mode of action and mechanisms underlying disease suppression in tomato plants.