We report the first gas-phase metallicity map of a distant galaxy measured with the James Webb Space Telescope (JWST). We use the NIRISS slitless spectroscopy acquired by the GLASS Early Release Science program to spatially resolve the rest-frame optical nebular emission lines in a gravitationally lensed galaxy at z = 3.06 behind the A2744 galaxy cluster. This galaxy (dubbed GLASS-Zgrad1) has stellar mass similar to 10(8.6)M(circle dot), instantaneous star formation rate similar to 8.6 M-circle dot yr(-1) (both corrected for lensing magnification), and global metallicity one-fourth solar. From its emission-line maps ([O III], H beta, H gamma, [Ne III], and [O II]), we derive its spatial distribution of gas-phase metallicity using a well-established forward-modeling Bayesian inference method. The exquisite resolution and sensitivity of JWST/NIRISS, combined with lensing magnification, enable us to resolve this z similar to 3 dwarf galaxy in greater than or similar to 50 resolution elements with sufficient signal, an analysis hitherto not possible. We find that the radial metallicity gradient of GLASS-Zgrad1 is strongly inverted (i.e., positive): Delta log (O/H)/Delta r = 0.165 +/- 0.023 dex kpc(-1). This measurement is robust at greater than or similar to 4 - sigma confidence level against known systematics. This positive gradient may be due to tidal torques induced by a massive nearby (similar to 15 kpc projected) galaxy, which can cause inflows of metal-poor gas into the central regions of GLASS-Zgrad1. These first results showcase the power of JWST wide-field slitless spectroscopic modes to resolve the mass assembly and chemical enrichment of low-mass galaxies in and beyond the peak epoch of cosmic star formation (z greater than or similar to 2). Reaching masses less than or similar to 10(9) M-circle dot at these redshifts is especially valuable to constrain the effects of galactic feedback and environment and is possible only with JWST's new capabilities.