Trichoderma aggressivum, a mycopathogen causing green mould disease, is a major problem in Agaricus bisporus cultivation due to crop loss, and resistance to chemical fungicides. There is an urgent need for novel biological ways to control mycopathogens without affecting the growth of A. bisporus. Bacteria from the mushroom-casing environment were identified and tested for antagonistic effect on T. aggressivum. Bacillus velezensis produced a large zone of inhibition and its supernatant inhibited the growth of T. aggressivum [−37%], and slightly stimulated A. bisporus growth [+2%]. Label free quantitative proteomic (LFQ) analysis of changes in the abundance of T. aggressivum proteins following exposure to B. velezensis supernatant indicated increased abundance of proteins associated with catabolic processing of amino acids (40-fold), amino oxidase proteins (14-fold), oxidoreductase proteins (13-fold, 4-fold) and hydrolases (3-fold). Proteins that decreased in relative abundance were antioxidants (29-fold), NTF2 domain containing protein (17-fold), 60S ribosomal protein L-13 (14-fold), glucoamylase proteins (13-fold), proteasome subunit proteins (11-fold) and other ribosomal proteins (9-fold). LFQ analysis revealed that exposing A. bisporus to B. velezensis supernatant led to a decrease in: prohibitin (13-fold, 6-fold), proteasomal proteins (11-fold), cytosolic adaptor domain containing protein (5-fold), aldehyde dehydrogenase (4-fold), ribosomal proteins (4- fold), DLH domain-containing protein (4-fold) and PKS_ER domain containing protein (3-fold). The results indicate that A. bisporus was not under stress upon contact with B. velezensis. Whereas a detrimental effect of B. velezensis on T. aggressivum is shown by inhibition of growth and damage-preventing proteins and increased abundance of proteins associated with stress.