Abstract We present a catalog of 23,790 extended low-surface-brightness galaxies (LSBGs) identified in from the first three years of imaging data from the Dark Energy Survey (DES). Based on a single-component Sérsic model fit, we define extended LSBGs as galaxies with g-band effective radii and mean surface brightness . We find that the distribution of LSBGs is strongly bimodal in (g − r) versus (g − i) color space. We divide our sample into red (g − i ≥ 0.60) and blue (g − i < 0.60) galaxies and study the properties of the two populations. Redder LSBGs are more clustered than their blue counterparts and are correlated with the distribution of nearby (z < 0.10) bright galaxies. Red LSBGs constitute ∼33% of our LSBG sample, and of these are located within 1° of low-redshift galaxy groups and clusters (compared to ∼8% of the blue LSBGs). For nine of the most prominent galaxy groups and clusters, we calculate the physical properties of associated LSBGs assuming a redshift derived from the host system. In these systems, we identify 41 objects that can be classified as ultradiffuse galaxies, defined as LSBGs with projected physical effective radii and central surface brightness . The wide-area sample of LSBGs in DES can be used to test the role of environment on models of LSBG formation and evolution.