This study investigates the antimicrobial and antibiofilm effects of 16 halogenated indoles (fluoro-, chloro-, bromo-, and iodo-) against the pathogenic *Escherichia coli* O157:H7 (EHEC) strain. The results show that the order of antibiofilm activity is chloroindoles > bromoindoles > indole > fluoroinoles. For example, 4-bromoindole and 5-bromoindole had minimum inhibitory concentrations (MICs) of 100 and 200 μg/mL, respectively, and at 20 μg/mL, they inhibited EHEC biofilm formation by more than 61% without affecting planktonic cell growth. At concentrations above their MICs, both compounds exhibited bactericidal activity. Scanning electron microscopy confirmed the antibiofilm effects, and both compounds reduced swimming and swarming motility and curli formation, which are crucial for EHEC biofilm formation. Quantitative structure–activity relationship analysis revealed that halogenation at positions C-4 or C-5 enhances antimicrobial activity, while substitution at C-7 is detrimental. The study suggests that halogenated indoles, particularly bromoindoles, have potential as antimicrobial and antibiofilm therapies against EHEC.This study investigates the antimicrobial and antibiofilm effects of 16 halogenated indoles (fluoro-, chloro-, bromo-, and iodo-) against the pathogenic *Escherichia coli* O157:H7 (EHEC) strain. The results show that the order of antibiofilm activity is chloroindoles > bromoindoles > indole > fluoroinoles. For example, 4-bromoindole and 5-bromoindole had minimum inhibitory concentrations (MICs) of 100 and 200 μg/mL, respectively, and at 20 μg/mL, they inhibited EHEC biofilm formation by more than 61% without affecting planktonic cell growth. At concentrations above their MICs, both compounds exhibited bactericidal activity. Scanning electron microscopy confirmed the antibiofilm effects, and both compounds reduced swimming and swarming motility and curli formation, which are crucial for EHEC biofilm formation. Quantitative structure–activity relationship analysis revealed that halogenation at positions C-4 or C-5 enhances antimicrobial activity, while substitution at C-7 is detrimental. The study suggests that halogenated indoles, particularly bromoindoles, have potential as antimicrobial and antibiofilm therapies against EHEC.