  We report the discovery and constrain the physical conditions of the
interstellar medium of the highest-redshift millimeter-selected dusty
star-forming galaxy (DSFG) to date, SPT-S J031132-5823.4 (hereafter
SPT0311-58), at $z=6.900 +/- 0.002$. SPT0311-58 was discovered via its 1.4mm
thermal dust continuum emission in the South Pole Telescope (SPT)-SZ survey.
The spectroscopic redshift was determined through an ALMA 3mm frequency scan
that detected CO(6-5), CO(7-6) and [CI](2-1), and subsequently confirmed by
detections of CO(3-2) with ATCA and [CII] with APEX. We constrain the
properties of the ISM in SPT0311-58 with a radiative transfer analysis of the
dust continuum photometry and the CO and [CI] line emission. This allows us to
determine the gas content without ad hoc assumptions about gas mass scaling
factors. SPT0311-58 is extremely massive, with an intrinsic gas mass of $M_{\rm
gas} = 3.3 \pm 1.9 \times10^{11}\,M_{\odot}$. Its large mass and intense star
formation is very rare for a source well into the Epoch of Reionization.
