Mediator, a central coregulator of transcription, has been identified as a large protein complex in eukaryotes ranging from yeast to man. It is therefore remarkable that Mediator has not yet been identified within the plant kingdom. Here we identify Mediator in a plant, Arabidopsis thaliana. The plant Mediator subunits typically show very low homology to other species, but our biochemical purification identifies 21 conserved and six A. thaliana-specific Mediator subunits. Most notably, we identify the A. thaliana proteins STRUWWELPETER (SWP) and PHYTOCHROME AND FLOWERING TIME 1 (PFT1) as the Med14 and Med25 subunits, respectively. These findings show that specific plant Mediator subunits are linked to the regulation of specialized processes such as the control of cell proliferation and the regulation of flowering time in response to light quality. The identification of the plant Mediator will provide new tools and insights into the regulation of transcription in plants.