Cosmological simulations predict that a large fraction of the baryonic mass of the Universe exists as 10^5-10^7 K diffuse, X-ray emitting gas, tracing low density filament and sheet-like structures exterior to massive clusters of galaxies. If present, this gas helps reconcile the current shortfall in observed baryon counts relative to the predictions of the standard Big Bang model. We present here the discovery and analysis of a 5-sigma significance half-degree filamentary structure, present in both I-band galaxy surface density and unresolved X-ray emission in a deep ROSAT PSPC field. The estimated diffuse X-ray emission component of this structure has a surface brightness of \~ 1.6 x 10^(-16) erg/s/cm^2/arcmin^2 (0.5- 2 keV), comparable to the predictions for inter-cluster gas and may represent a direct detection of this currently unconfirmed baryonic component. Comment: 10 pages, uses aaspp4.sty, 2 figures included, accepted ApJ Letters