We have derived high-spatial-resolution metallicity maps covering ∼42 deg2 across the Small Magellanic Cloud (SMC) in an attempt to understand its metallicity distribution and gradients up to a radius of ∼4°. Using the near-infrared VISTA Survey of the Magellanic Clouds, our data cover a thrice larger area compared with previous studies. We identify red giant branch (RGB) stars in spatially distinct Y, (Y-Ks) colour-magnitude diagrams. In any of our selected subregions, the RGB slope is used as an indicator of the average metallicity, based on calibration to metallicity using spectroscopic data. The metallicity distribution across the SMC is unimodal and can be fitted by a Gaussian distribution with a peak at [Fe/H] =-0.97 dex (σ[Fe/H] = 0.05 dex). We find evidence of a shallow gradient in metallicity (-0.031 ± 0.005 dex deg-1) from the Galactic Centre to radii of 2°-2°.5 followed by a flat metallicity trend from ∼3°.5 to 4°. We find that the SMC's metallicity gradient is radially asymmetric. It is flatter towards the east than to the west, hinting at mixing and/or distortion of the spatial metallicity distribution (within the inner 3°), presumably caused by tidal interactions between the Magellanic Clouds.