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We have discovered a new cluster of galaxies, RX~J0018.3+1618, in a deep ROSAT PSPC pointing of the distant cluster CL0016+16. The X-ray flux of the new cluster is $8 \times 10^{-14}$ ergs s$^{-1}$ cm$^{-2}$ in the 0.2--2 keV band, which is about 10\% of the X-ray flux of CL0016+16. We carried out optical spectroscopy at the Multiple Mirror Telescope and measured the redshifts of 10 galaxies in the direction of RX~J0018.3+1618. Eight galaxies lie at a redshift of $\sim$0.55. The difference in mean radial velocity between the new cluster (z=0.5498) and CL0016+16 (z=0.5455) is comparable to the internal velocity dispersions of the clusters. The angular separation between them is $10^\prime$ corresponding to a linear distance of 4.4 Mpc for a Hubble constant of 50 km s$^{-1}$ Mpc$^{-1}$ and $q_0 = 0.5$. We investigate the likelihood that these clusters are bound and speculate on the implications of this discovery for models of large scale structure.
