Allionia, commonly known as windmills or trailing four o'clock, is a genus of two plant species widespread in the Western Hemisphere.

The plants are finely pubescent annuals or short-lived perennials, with trailing, recumbent stems up to one metre in length, often threading through other vegetation.

The individual flowers are bisexual and bilaterally symmetric with a distinct oblique funnel.

[1] Molecular evidence supports Allionia as sister to the least inclusive clade containing both Boerhavia and Cyphomeris.

Carl Linnaeus named the genus after Italian botanist Carlo Allioni (1725–1804).