Using moderate-resolution optical spectra from 58 background Lyman-break galaxies and quasars at z~ 2.3-3 within a 11'.5 x 13'.5 area of the COSMOS field (~ 1200 deg super(-2) projected area density or ~2.4h super(-1) Mpc mean transverse separation), we reconstruct a 3D tomographic map of the foreground Ly alpha forest absorption at 2.2 < z< 2.5 with an effective smoothing scale of epsilon sub(3D)approximate 2.5 h super(-1) Mpc comoving. Comparing with 61 coeval galaxies with spectroscopic redshifts in the same volume, we find that the galaxy positions are clearly biased toward regions with enhanced intergalactic medium (IGM) absorption in the tomographic map. We find an extended IGM overdensity with deep absorption troughs at z= 2.45 associated with a recently discovered galaxy protocluster at the same redshift. Based on simulations matched to our data, we estimate the enclosed dark matter mass within this IGM overdensity to be M sub(dm)(z=2.45 )=(1.1 + or - 0.6) x 10 super(14) h super(-1)M sub(middot in circle), and argue based on this mass and absorption strength that it will form at least one z~ 0 galaxy cluster with M (z=0)=(3 + or - 1.5) x 10 super(14)h super(-1)M sub(middot in circle), although its elongated nature suggests that it will likely collapse into two separate clusters. We also point out a compact overdensity of six MOSDEF galaxies at z= 2.30 within a r~ 1 h super(-1) radius and Delta z~ 0.006, which does not appear to have a large associated IGM overdensity. These results demonstrate the potential of Ly alpha forest tomography on larger volumes to study galaxy properties as a function of environment, as well as revealing the large-scale IGM overdensities associated with protoclusters or other features of large-scale structure.