We report a highly significant ($>10sigma$) spatial correlation between galaxies with $S_{350murm m}ge 30,$mJy detected in the equatorial fields of the textsl{Herschel} Astrophysical Terahertz Large Area Survey (H-ATLAS) with estimated redshifts $gtrsim 1.5$, and SDSS or GAMA galaxies at $0.2le zle 0.6$. The significance of the cross-correlation is much higher than those reported so far for samples with non-overlapping redshift distributions selected in other wavebands. Extensive, realistic simulations of clustered sub-mm galaxies amplified by foreground structures confirm that the cross-correlation is explained by weak gravitational lensing ($mu<2$). The simulations also show that the measured amplitude and range of angular scales of the signal are larger than can be accounted for by galaxy-galaxy weak lensing. However, for scales $lesssim 2,$arcmin, the signal can be reproduced if SDSS/GAMA galaxies act as signposts of galaxy groups/clusters with halo masses in the range $10^{13.2}$--$10^{14.5} M_{odot}$. The signal detected on larger scales appears to reflect the clustering of such halos.
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