A model of spatiotemporal signal processing by the cone–horizontal cell circuit in the primate outer retina is developed and validated using measurements on the H1 horizontal cell from the literature. The model extends an earlier temporal model that mainly addressed the regulation of sensitivity by the cones. Three elements are added to the earlier model to describe the full spatiotemporal processing by horizontal cells. First, the feedback gain from horizontal cells to cones is made adaptive, depending on field size. Second, the spatial filtering by the horizontal dendritic tree is modeled as a two-component spatial filter. Third, an adaptive temporal low-pass filter is added, also depending on field size. The resulting model adequately describes all available measurements on spatiotemporal processing in macaque H1 cells. The adaptive feedback gain is argued to contribute to negative afterimages and chromatic adaptation in human vision.