A vouchered species list of 275 vascular plants was compiled for the Sun Lakes State Park, a 4,027-acre park located in Grant County, Washington, east of the Cascades in the Columbia Basin. Native flora in the park makes up 87% of recorded species. Ninety-seven new collections were made in the spring of 2010, contributing 42 new species to the previous collections housed at the herbaria of University of Washington Burke Museum and the Evergreen State College. The Missoula floods of the last ice age uniquely carved the landscape approximately 15,000 years ago, leaving a mosaic of wetlands, shallow soils scoured from flood action, and large gravel bars, resulting in a heterogeneous landscape of high floristic diversity. The vegetation is dominated primarily by big sagebrush (Artemisia tridentata) and various perennial bunchgrasses such as bluebunch wheatgrass (Agropyron spicatum). The park represents a relatively intact portion of the sagebrush steppe, a landscape that generally has been heavily impacted by development and agriculture.