  Context: Trumpler 23 is a moderately populated, intermediate-age open cluster
within the solar circle at a Rgc ~6 kpc. It is in a crowded field very close to
the Galactic plane and the color-magnitude diagram shows significant field
contamination and possible differential reddening; it is a relatively
understudied cluster for these reasons, but its location makes it a key object
for determining Galactic abundance distributions.
  Aims: New data from the Gaia-ESO Survey enable the first ever radial velocity
and spectroscopic metallicity measurements for this cluster. We aim to use
velocities to isolate cluster members, providing more leverage for determining
cluster parameters.
  Methods: Gaia-ESO Survey data for 167 potential members have yielded radial
velocity measurements, which were used to determine the systemic velocity of
the cluster and membership of individual stars. Atmospheric parameters were
also used as a check on membership when available. Literature photometry was
used to re-determine cluster parameters based on radial velocity member stars
only; theoretical isochrones are fit in the V, V-I diagram. Cluster abundance
measurements of ten radial-velocity member stars with high-resolution
spectroscopy are presented for 24 elements. These abundances have been compared
to local disk stars, and where possible placed within the context of literature
gradient studies.
  Results: We find Trumpler 23 to have an age of 0.80 +/- 0.10 Gyr, significant
differential reddening with an estimated mean cluster E(V-I) of 1.02
+0.14/-0.09, and an apparent distance modulus of 14.15 +/- 0.20. We find an
average cluster metallicity of [Fe/H] = 0.14 +/- 0.03 dex, a solar [alpha/Fe]
abundance, and notably subsolar [s-process/Fe] abundances.
