The Santa Catalina Mountains are located in Pima and Pinal counties in southeastern Arizona. The study area is defined as approximately 259,000 acres (104,813 hectares) or 405 mi2 (1,048 km2), spanning an elevational gradient of 6,457 ft (1,968 m). Located on the northwestern edge of the Madrean-influenced sky island complex of southeastern Arizona and northern Sonora, highly diverse plant communities range from Sonoran Desert to subalpine forest. A total of 380 days of field work were conducted between 2007–2017, including extensive exploration of the remote east side of the mountains. The vascular flora includes 1,360 taxa in 127 families and is currently the largest of any range in southern Arizona. Non-native plants are represented by 167 taxa and comprise 12.3% of the total flora. The three largest plant families are Asteraceae, Poaceae and Fabaceae, with 213, 187 and 107 taxa respectively. Euphor-bia, Muhlenbergia and Dalea are the largest genera with 24, 22 and 16 species. A total of 375 taxa are found on lime-stone or dolomitic substrates. There are 69 historically collected taxa that have not been seen or collected in 55 years, which are excluded from this checklist. New additions to the vascular flora are vouchered at the University of Arizona Herbarium. A checklist of 169 non-vascular plants from 36 families, based on over 1,150 collections from 18 national herbaria, is included. The floristic diversity of this sky island represents nearly a third of the entire state flora, while occupying less than half a percent of the state’s area. Geographic location, elevational gradient, geological diversity, and a high percentage of species found at the edge of their ranges contribute to the rich diversity of this unique mountain range.Monsoon storms cover the west side of the range.