  We find a new supersymmetric solution of type IIB supergravity which is
holographically dual to the planar limit of the four-dimensional
$\mathcal{N}=2^*$ supersymmetric Yang-Mills theory on $S^4$. We study a probe
fundamental string in this background which is dual to a supersymmetric Wilson
loop in the $\mathcal{N}=2^*$ theory. Using holography we calculate the
expectation value of this line operator to leading order in the 't Hooft
coupling. The result is a non-trivial function of the mass parameter of the
$\mathcal{N}=2^*$ theory that precisely matches the result from supersymmetric
localization.
