Purpose: Immunogenicity of Covid-19 vaccines may be negatively impacted by anti-cancer treatment. The management of primary brain tumors (PBTs) routinely includes temozolomide and steroids, which are immune-suppressive. We aimed to determine the rate of seropositivity in PBT patients following receipt of two doses of the BNT162b2 vaccine. Methods: We prospectively evaluated IgG levels against SARS-CoV-2 spike protein in 17 PBT patients following two doses of the BNT162b2 vaccine. IgG levels were collected at two time points: T1—after a median of 44 days from the second vaccine dose and T2—after a median of 130 days from the second dose. Titers were compared against a group of healthy controls (HC) comprised of patients’ family members. Results: At T1, 88.2% (15/17) of PBT patients achieved seroconversion, compared with 100% (12/12) of HCs. Median IgG titer was significantly lower in the PBT group (1908 AU/mL vs 8,198 AU/mL; p = 0.002). At T2, 80% (12/15) of PBT patients seroconverted, compared to 100% (10/10) of HCs. Median IgG titer remained significantly lower in the PBT group (410 AU/mLvs 1687 AU/mL; p = 0.002). During the peri-vaccination period, 15 patients received systemic treatment and 8 patients were treated with corticosteroids. All 3 patients who failed to seroconvert at T2 were treated with corticosteroids. In a univariate analysis, steroid use was negatively associated with antibody titer. Conclusion: Most PBT patients successfully seroconvert following two doses of the BNT162b2 vaccine, albeit with lower antibody titer compared to HCs. Steroid use during the vaccination period is associated with lower titer.