Karl Andreas Geyer (30 November 1809 – 21 November 1853) was a German botanist who was a native of Dresden.
As a teenager, Geyer worked as an apprentice-gardener in Zabeltitz, and in 1830 became an assistant at the botanical gardens in Dresden. From 1835 to 1844 he performed botanical studies on several expeditions within the United States. In 1838-40 he worked as a botanist in the Upper Midwest for geographer Joseph Nicollet (1786-1843), and in 1841-42 collected plants in Illinois, Missouri and the Iowa Territory for botanist Georg Engelmann (1809-1884).
Afterwards he joined explorer William Drummond Stewart (1795-1871) on an expedition through the present-day states of Nebraska and Wyoming. Eventually, Geyer parted company with Drummond, and performed extensive botanical research in what would later be known as the Oregon Territory. In 1845 he returned to Germany, where he purchased land in Meissen and started a nursery.
Geyer has several plant species named after him, including Allium geyeri (Geyer's onion) and Euphorbia geyeri (Geyer's spurge). His botanical collection of nearly 10,000 specimens was acquired by the Royal Botanical Gardens at Kew.