Systems and methods for sealing bolted flange joints to stop or prevent leakage are disclosed. The system includes a sealant injection nut. The sealant injection nut generally comprises a threaded nut configured to be threaded onto a stud bolt of a flange joint. The nut is configured to inject sealant into the internal passages of the flange joint. The nut includes a passageway drilled through the sidewall. Fitted to the passageway is a high-pressure sealant-compatible injection fitting and an internal check-valve. Provided within the central cavity of the nut, at the flange-facing end, is a groove that facilitates the flow of sealant injected through the passageway into the passages of the bolted flange. A method is also provided for manufacturing the sealant injection nut. A method is also provided for repairing leaking bolted flange joints using the sealant injection nut.