  The nucleon axial coupling constant $g_A$ is calculated in the presence of an
external uniform magnetic field using finite energy sum rules. The correlation
function of proton, neutron and axial-vector currents is calculated both the
hadronic and the QCD sector. Once the axial contribution in the form factor is
isolated, a double sum rule is considered, i.e., the usual QCD contour
integration for the external moments of both the proton and neutron current.
The effects of the external magnetic field come mainly through the in-medium
current-nucleon coupling constants $\lambda_N(B)$ and the hadronic thresholds
$s_0(B)$ provided by the nucleon-nucleon correlators. As a result, the axial
coupling constant decreases in the presence of the magnetic field.
