The clerodane diterpenoids 16-oxo-cleroda-3, 13(14) E-diene-15 oic acid (1) and kolavenic acid (2) isolated from Polyalthia longifolia var. pendula (Linn.) were previously reported for their antimicrobial activity. Thus present study was designed to investigate the biofilm inhibiting potential of these diterpenoids (1-2) and five new lactone derivatives (3-7) of 1 against methicillin resistant Staphylococcus aureus (MRSA), Streptococcus mutans, Klebsiella pneumoniae and Proteus mirabilis. Compounds 1 and 3 at 10-20 μg/mL were found to be bacteriostatic and significantly reduced the biofilm formation and metabolically active cells of MRSA and S. mutans up to 90%. Parental diterpenoid (1) at 10 and 16 μg/mL significantly eradicated the preformed biofilm of both pathogens. Both the compounds also delayed acid production at minimum inhibitory (MIC) and sub-inhibitory concentrations (sub MIC). Florescence and scanning electron microscopy further confirms the biofilm inhibiting potential of compounds 1 and 3 and displayed disrupted biofilms at MIC and sub MIC levels. The gene expression of MRSA and S. mutans responsible for biofilm formation was also altered. Moreover, the observed anti-virulence properties and delayed bacterial growth after 10 min of exposure to the test compounds 1 and 3 make them a promising class of antibiofilm agents.