Rationale & Objective Recent studies showed that antibody titers after vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the dialysis population are diminished as compared to the general population, suggesting the possible value of a third booster dose. We aimed to characterize the humoral response after three doses of the BNT162b2 vaccine in patients treated with either maintenance hemodialysis (HD) or peritoneal dialysis (PD). Study Design Case series. Setting & Participants 69 French patients (38 HD and 31 PD) treated at a single center who received three doses of the BNT162b2 vaccine. Findings Humoral response was evaluated using plasma levels of anti-SARS-CoV-2 spike protein S1 immunoglobulin measured after the second dose and at least three weeks after the third dose of the BNT162b2 vaccine. Patients (median age 68 [IQR, 53-76] years, 65% men) had a median anti-S1 antibody level of 284 [IQR, 83-1190] AU/mL after the second dose, and 7554 [IQR, 2268-11736] AU/mL after the third dose. Three patients were non-responders (anti-S1 antibody level < 0.8 AU/mL) and 12 were weak responders (anti-S1 antibody level 0.8-50 AU/mL) after the second vaccine dose. After the third dose, one of the three initial non-responders produced anti-spike antibody and all the 12 initial weak responders increased their antibody levels. Patients with a greater increase in anti-S1 antibody levels after a third dose had lower antibody levels after the second dose, and a longer time interval between the second and the third dose. Adverse events did not seem to be more common or severe following a third vaccine dose. Limitations Observational study, small sample size. Relationship between antibody levels and clinical outcomes is not well understood. Conclusions A third dose of the BNT162b2 vaccine substantially increased antibody levels in patients receiving maintenance dialysis and appeared to be as well tolerated as a second dose.