The fungal pathogen Lecanicillium fungicola causes dry bubble disease in Agaricus bisporus cultivation and affected mushrooms significantly reduce the yield and revenue for mushroom growers. Biocontrol agents may represent an alternative and more environmentally friendly treatment option to help control dry bubble on mushroom farms. Serenade ® is a commercially available biocontrol product used for disease treatment in plant crops. In this work, the in vitro response of L. fungicola to the bacterial strain active in Serenade, Bacillus velezensis (QST 713) and a newly isolated B. velezensis strain (Kos) was assessed. B. velezensis (QST713 and Kos) both produced zones of inhibition on plate cultures of L. fungicola, reduced the mycelium growth in liquid cultures and damaged the morphology and structure of L. fungicola hyphae. The proteomic response of the pathogen against these biocontrol strains was also investigated. Proteins involved in growth and translation such as 60S ribosomal protein L21-A (−32- fold) and 40S ribosomal protein S30 (−17-fold) were reduced in abundance in B. velezensis QST 713 treated samples, while proteins involved in a stress response were increased (norsolorinic acid reductase B (47-fold), isocitrate lyase (11-fold) and isovaleryl-CoA dehydrogenase (8-fold). L. fungicola was found to have a similar proteomic response when exposed to B. velezensis (Kos). This work provides information on the response of L. fungicola to B. velezensis (QST 713) and indicates the potential of B. velezensis Kos as a novel biocontrol agent.