  We present a catalog of 689 galaxy cluster candidates detected at
significance $\xi>4$ via their thermal Sunyaev-Zel'dovich (SZ) effect signature
in 95 and 150 GHz data from the 500-square-degree SPTpol survey. We use optical
and infrared data from the Dark Energy Camera and the Wide-field Infrared
Survey Explorer (WISE) and \spitzer \ satellites, to confirm 544 of these
candidates as clusters with $\sim94\%$ purity. The sample has an approximately
redshift-independent mass threshold at redshift $z>0.25$ and spans $1.5 \times
10^{14} < M_{500c} < 9.1 \times 10^{14}$ $M_\odot/h_{70}$ \ and
$0.03<z\lesssim1.6$ in mass and redshift, respectively; 21\% of the confirmed
clusters are at $z>1$. We use external radio data from the Sydney University
Molonglo Sky Survey (SUMSS) to estimate contamination to the SZ signal from
synchrotron sources. The contamination reduces the recovered $\xi$ by a median
value of 0.032, or $\sim0.8\%$ of the $\xi=4$ threshold value, and $\sim7\%$ of
candidates have a predicted contamination greater than $\Delta \xi = 1$. With
the exception of a small number of systems $(<1\%)$, an analysis of clusters
detected in single-frequency 95 and 150 GHz data shows no significant
contamination of the SZ signal by emission from dusty or synchrotron sources.
This cluster sample will be a key component in upcoming astrophysical and
cosmological analyses of clusters. The SPTpol millimeter-wave maps and
associated data products used to produce this sample are available at
https://pole.uchicago.edu/public/data/sptpol_500d_clusters/index.html, and the
NASA LAMBDA website. An interactive sky server with the SPTpol maps and Dark
Energy Survey data release 2 images is also available at NCSA
https://skyviewer.ncsa.illinois.edu.
