  We present the first detection of weak gravitational shear at infrared
wavelengths, using observations of the lensing cluster Abell 1689, taken with
the SofI camera on the ESO-NTT telescope. The imprint of cluster lenses on the
shapes of the background galaxy population has previously been harnessed at
optical wavelengths, and this gravitational shear signal enables cluster mass
distributions to be probed, independent of whether the matter is luminous or
dark. At near-infrared wavelengths, the spectrophotometric properties of
galaxies facilitate a clean selection of background objects for use in the
lensing analysis. A finite-field mass reconstruction and application of the
aperture mass (Map) statistic are presented. The probability that the peak of
the Map detection S/N~5, arises from a chance alignment of background sources
is only ~4.5*10^-7. The velocity dispersion of the best-fit singular isothermal
sphere model for the cluster is sigma_1D=1030^{+70}_{-80} km/s, and we find a
K-band mass-to-light ratio of ~40 M_solar/L_solar inside a 0.44 Mpc radius.
