Background <p>Toxoplasmosis, caused by <i>Toxoplasma gondii</i>, affects a significant proportion of the global population. While typically asymptomatic in healthy individuals due to effective immune responses, immunocompromised individuals, such as cancer patients undergoing chemotherapy, are at risk of developing acute toxoplasmosis due to the reactivation of latent infection. This study aims to investigate the seroprevalence of latent <i>T. gondii</i> infection among cancer patients who were registered with the Iranian National Registry Center for Toxoplasmosis (INRCT), before and after chemotherapy, and to determine the relationship between chemotherapy and IgG antibody responses.</p> Patients and Methods <p>This cross-sectional study investigated the seroprevalence of <i>T. gondii</i> infection and its relationship with chemotherapy in 107 cancer patients with solid organ tumors. Participants were recruited from oncology centers in Sari, northern Iran, between 2023 and 2024 and were assessed for anti-<i>T. gondii</i> IgG and IgM antibodies using ELISA before and three weeks after chemotherapy initiation.</p> Results <p>At baseline, 80.4% of patients were IgG positive, indicating prior <i>T. gondii</i> exposure. Three weeks post-chemotherapy, 28.92% showed a significant increase in IgG titers. While age was significantly associated with <i>Toxoplasma</i> infection at baseline (<i>P</i> &lt; 0.05), no significant associations were found between demographic or clinical characteristics and IgG seropositivity or titer changes post-chemotherapy. Breast cancer patients exhibited the highest prevalence of <i>Toxoplasma</i> IgG.</p> Conclusions <p>These findings suggest a high prevalence of latent toxoplasmosis in cancer patients, with chemotherapy potentially influencing IgG titers in a subset of individuals, independent of other factors examined. Further research is needed to elucidate the underlying mechanisms and clinical implications of these observations.</p>