  The Galactic Center supermassive black hole Sagittarius A* (Sgr A*) is one of
the most promising targets to study the dynamics of black hole accretion and
outflow via direct imaging with very long baseline interferometry (VLBI). At
3.5 mm (86 GHz), the emission from Sgr A* is resolvable with the Global
Millimeter VLBI Array (GMVA). We present the first observations of Sgr A* with
the phased Atacama Large Millimeter/submillimeter Array (ALMA) joining the
GMVA. Our observations achieve an angular resolution of ~87{\mu}as, improving
upon previous experiments by a factor of two. We reconstruct a first image of
the unscattered source structure of Sgr A* at 3.5 mm, mitigating effects of
interstellar scattering. The unscattered source has a major axis size of 120
$\pm$ 34{\mu}as (12 $\pm$ 3.4 Schwarzschild radii), and a symmetrical
morphology (axial ratio of 1.2$^{+0.3}_{-0.2}$), which is further supported by
closure phases consistent with zero within 3{\sigma}. We show that multiple
disk-dominated models of Sgr A* match our observational constraints, while the
two jet-dominated models considered are constrained to small viewing angles.
Our long-baseline detections to ALMA also provide new constraints on the
scattering of Sgr A*, and we show that refractive scattering effects are likely
to be weak for images of Sgr A* at 1.3 mm with the Event Horizon Telescope. Our
results provide the most stringent constraints to date for the intrinsic
morphology and refractive scattering of Sgr A*, demonstrating the exceptional
contribution of ALMA to millimeter VLBI.
