The 1055 ha Strouds Run State Park is located in the southern unglaciated Allegheny Plateau physiographic province in southeastern Ohio. The park has a rugged landscape with steep-sided, narrowly rounded hills and narrow, V-shaped valleys. The vegetation consists mainly of oak-dominated and mixed mesophytic forests but also includes marshes, rock outcrops, and three small prairie-like openings. This study documents the terrestrial vascular flora of Strouds Run State Park and compares it with the flora of the same area reported in 1957. Between 2003 and 2005, 638 species in 367 genera and 108 families were found in the park. This total includes five State-listed, “potentially threatened” taxa: Arabis hirsuta var. adpressipilis, Carex cumberlandensis, Corallorhiza wisteriana, Cystopteris tennesseensis, and Spiranthes ovalis. The number of species increased from 534 to 638 over the course of 49 years, while the percentage of alien species declined from 23.6% to 18.7%. However, the number and abundance of alien species that are invasive have increased greatly since 1957.