Lomatium cookii is a rare species of flowering plant in the carrot family known by the common names Cook's lomatium and agate desertparsley. It is endemic to Oregon in the United States, where it grows in only two valleys. It is a federally listed endangered species.

This plant was first collected in 1981 during a survey for the rare Limnanthes floccosa ssp. grandiflora, the big-flowered woolly meadowfoam. It was described as a new species in 1986. The plant grows only in the Agate Desert of Jackson County and the Illinois Valley of Josephine County in southwestern Oregon. It occurs in vernally wet habitat types, including vernal pools and adjacent mounds and wet floodplains. One population of the plant occurs at Rogue Valley International-Medford Airport, where soil has been deposited over hardpan such that conditions are similar to vernal pool margins, allowing it to take hold there. Other plants that can be found in the vernal pool and floodplain habitat types include Limnanthes floccosa (woolly meadowfoam), Alopecurus geniculatus (water foxtail), Deschampsia danthonioides (annual hairgrass), Danthonia californica (California oatgrass), Poa scabrella (pine bluegrass), and Brodiaea spp. (brodiaeas or clusterlilies).

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