  If gamma-ray bursts are at cosmological distances, they must be
gravitationally lensed occasionally. The detection of lensed images with
millisecond-to-second time delays provides evidence for intermediate-mass black
holes, a population that has been difficult to observe. Several studies have
searched for these delays in gamma-ray burst light curves, which would indicate
an intervening gravitational lens. Among the $\sim 10^4$ gamma-ray bursts
observed, there have been a handful of claimed lensing detections, but none
have been statistically robust. Here we present a Bayesian analysis identifying
gravitational lensing in the light curve of GRB950830. The inferred lens mass
depends on the unknown lens redshift $z_l$, and is given by $(1+z_l)M_l =
5.5^{+1.7}_{-0.9}\times 10^4 $ M$_\odot$ (90% credibility), which we interpret
as evidence for an intermediate-mass black hole. The most probable
configuration, with a lens redshift $z_l\sim 1$ and a gamma-ray burst redshift
$z_s\sim 2$, yields a present day number density of $n_\text{imbh}\approx
2.3^{+4.9}_{-1.6}\times10^{3} \text{ Mpc}^{-3}$ (90% credibility) with a
dimensionless energy density $\Omega_\text{imbh} \approx
4.6^{+9.8}_{-3.3}\times10^{-4}$. The false alarm probability for this detection
is $\sim0.6\%$ with trial factors. While it is possible that GRB950830 was
lensed by a globular cluster, it is unlikely since we infer a cosmic density
inconsistent with predictions for globular clusters $\Omega_\text{gc} \approx 8
\times 10^{-6}$ at 99.8% credibility. If a significant intermediate-mass black
hole population exists, it could provide the seeds for the growth of
supermassive black holes in the early Universe.
