Mirabilis rotundifolia is a species of flowering plant in the four o'clock family known by the common name roundleaf four o'clock. It is endemic to Colorado in the United States, where it is limited to the south-central part of the state. It occurs in the Arkansas River Valley in Fremont, Pueblo, and Las Animas Counties. This plant grows on the Smoky Hill Member of the Niobrara Formation, generally on outcrops of shale. The habitat is mostly barren terrain in shrubland and woodland ecosystems. Associated species include Frankenia jamesii.