Orobanche cernua Loefl. is one of the most destructive parasitic weeds affecting tomato production. In the present study, the effects of four resistance-inducing compounds, namely isonicotinic acid (INA), β-aminobutyric acid (BABA), riboflavin, and jasmonic acid, on the suppression of O. cernua and the induction of defense responses in tomato were investigated. Host plant growth parameters, broomrape growth characteristics, defense-related enzyme activities, total phenolic content, lignin accumulation, and histological changes in tomato roots were evaluated. The results showed that all resistance inducers, reduced the growth and development of O. cernua while improving the growth of the host plant; however, isonicotinic acid exhibited the greatest efficacy. INA significantly reduced the number tubercles as well as dry weight of broomrape and markedly enhanced tomato growth parameters. In addition, INA treatment significantly increased the activities of phenylalanine ammonia-lyase (PAL) and peroxidase (POX), as well as total phenolic content and lignin accumulation in tomato roots. Histological examinations further demonstrated that haustorial penetration and vascular establishment of the parasite were markedly restricted in the roots of INA-treated plants compared with the untreated control. Overall, the findings indicate that isonicotinic acid suppresses the establishment and development of O. cernua by strengthening both the biochemical and structural defense responses of tomato plants. Therefore, INA has considerable potential as an environmentally compatible resistance inducer for incorporation into integrated broomrape management programs. Nevertheless, further field investigations and molecular studies are required to confirm its efficacy under field conditions and to elucidate the precise defense mechanisms underlying INA-induced resistance.