We identify a surface within the NASA Phoenix landing site Area D characterized by boulder-topped, polygonally-patterned mounds comparable in radius and cross-sectional morphology to terrestrial sublimation polygons found in Antarctica. Both Martian and Antarctic polygons display topographic asymmetry, with shallow equator-facing slopes and steep pole-facing slopes, interpreted to indicate insolation-dependent, differential sublimation of buried ice. On the basis of morphological similarities, we classify the Phoenix Box 1 polygons as sublimation polygons. Terrestrial sublimation polygons form where ice volumes exceeding available pore space occur in the shallow subsurface and where near-surface conditions are too cold and dry to permit the development of saturated active layers: conditions comparable to those recently modeled for Mars in the Phoenix landing site Area D, Box 1. The identification of sublimation polygons on Mars would provide direct evidence for shallow, ground ice exceeding pore-ice volumes and shed light on the emplacement mechanism of this ice.