  We report on a spectroscopic determination of the atmospheric parameters and
chemical abundance of the parent star of the recently discovered transiting
planet {TrES-2}. A detailed LTE analysis of a set of \ion{Fe}{1} and
\ion{Fe}{2} lines from our Keck spectra yields $T_\mathrm{eff} = 5850\pm 50$ K,
$\log g = 4.4\pm 0.1$, and [Fe/H] $= -0.15\pm 0.10$. Several independent checks
(e.g., additional spectroscopy, line-depth ratios) confirm the reliability of
our spectroscopic $T_\mathrm{eff}$ estimate. The mass and radius of the star,
needed to determine the properties of the planet, are traditionally inferred by
comparison with stellar evolution models using $T_\mathrm{eff}$ and some
measure of the stellar luminosity, such as the spectroscopic surface gravity
(when a trigonometric parallax is unavailable, as in this case). We apply here
a new method in which we use instead of $\log g$ the normalized separation
$a/R_\star$ (related to the stellar density), which can be determined directly
from the light curves of transiting planets with much greater precision. With
the $a/R_\star$ value from the light curve analysis of Holman et al.
\citeyearpar{holman07b} and our $T_\mathrm{eff}$ estimate we obtain $M_\star =
0.980\pm0.062 M_\odot$ and $R_\star = 1.000_{-0.033}^{+0.036} R_\odot$, and an
evolutionary age of $5.1^{+2.7}_{-2.3}$ Gyr, in good agreement with other
constraints based on the strength of the emission in the \ion{Ca}{2} H & K line
cores, the Lithium abundance, and rotation. The new stellar parameters yield
improved values for the planetary mass and radius of $M_p = 1.198 \pm 0.053
M_\mathrm{Jup}$ and $R_p = 1.220^{+0.045}_{-0.042} R_\mathrm{Jup}$, confirming
that {TrES-2} is the most massive among the currently known nearby ($d\lesssim
300$ pc) transiting hot Jupiters. [Abridged]
