Vaccine‐induced SARS‐CoV‐2 antibody responses are attenuated in solid organ transplant recipients (SOTRs) and breakthrough infections are more common. Additional SARS‐CoV‐2 vaccine doses increase anti‐spike IgG in some SOTRs, but it is uncertain whether neutralization of variants of concern (VOCs) is enhanced. We tested 47 SOTRs for clinical and research anti‐spike IgG, pseudoneutralization (ACE2 blocking), and live‐virus neutralization (nAb) against VOCs before and after a third SARS‐CoV‐2 vaccine dose (70% mRNA, 30% Ad26.COV2.S) with comparison to 15 healthy controls after two mRNA vaccine doses. We used correlation analysis to compare anti‐spike IgG assays and focused on thresholds associated with neutralization. A third SARS‐CoV‐2 vaccine dose increased median total anti‐spike (1.6‐fold), pseudoneutralization against VOCs (2.5‐fold vs. Delta), and neutralizing antibodies (1.4‐fold against Delta). However, neutralization activity was significantly lower than healthy controls (p < .001); 32% of SOTRs had zero detectable nAb against Delta after third vaccination compared to 100% for controls. Correlation with nAb was seen at anti‐spike IgG >4 Log10(AU/ml) on the Euroimmun ELISA and >4 Log10(AU/ml) on the MSD research assay. These findings highlight benefits of a third vaccine dose for some SOTRs and the need for alternative strategies to improve protection in a significant subset of this population. Neutralizing antibodies against SARS‐CoV‐2 variants of concern increase in solid organ transplant recipients after a third dose of SARS‐CoV2 vaccine but remain significantly lower iin comparison to healthy controls.