Red pitahaya and red spinach contain betacyanins which result in their red colour. These water soluble nitrogenous pigments exhibit a range of bioactivities. Sub-fractionated plant extracts from the two plants were tested for biofilm inhibitory activity against 5 Staphylococcus aureus strains and 4 Pseudomonas aeruginosa strains. The minimum biofilm inhibitory concentration (MBIC) was determined. Among the S. aureus strains, the lowest MBIC (0.313 mg/mL) was against strain ATCC 33591 for both plant extracts. Red spinach extract also showed the same MBIC (0.313 mg/mL) against S. aureus ATCC 43300, ATCC 6538P and ATCC 29213. Among the P. aeruginosa strains, the lowest MBIC (0.625 mg/mL) was against strain ATCC 9027 for red pitahaya and against strain ATCC 27853 for red spinach. Growth curve analysis of both bacterial species showed that they were not affected by either plant extracts at concentrations up to 2.5 mg/mL. The effect of plant extracts on cell surface hydrophobicity was assessed using bacterial adherence to hydrocarbon assay. Red pitahaya significantly (p<0.05) reduced hydrophobicity of all S. aureus strains, whereas red spinach significantly (p<0.05) reduced hydrophobicity of all P. aeruginosa strains. Overall, red pitahaya was more effective on P. aeruginosa, whereas red spinach was more effective on S. aureus. The extracts were also tested for the ability to inhibit bacterial attachment to polystyrene and glass. Numbers of S. aureus attached to polystyrene were reduced by 0.6-1.1 log CFU/cm2 by red spinach and by 0.5-0.8 log CFU/cm2 by red pitahaya extracts. Numbers of P. aeruginosa attached to polystyrene were reduced by 1.0-1.7 log CFU/cm2 by red spinach and by 0.5-0.6 log CFU/cm2 by red pitahaya extracts. The reduction in adherence to polystyrene was greater as compared to glass. The results suggest betacyanins might act as an anti-biofilm agent against the initial step of biofilm formation, particularly on hydrophobic surfaces.