Stomata are highly specialized organs which consist of pairs of guard cells and regulate gas and water vapor exchange in plants [1–3]. While early stages of guard cell differentiation have been described [4–10] and were interpreted in analogy to processes of cell type differentiation in animals [11], the downstream development of functional stomatal guard cells remains poorly understood. We have isolated an Arabidopsis mutant, scap1 (stomatal carpenter 1), that develops irregularly shaped guard cells and lacks the ability to control stomatal aperture, including CO2-induced stomatal closing and light-induced stomatal opening. SCAP1 was identified as a plant-specific Dof-type transcription factor expressed in maturing guard cells but not in guard mother cells. SCAP1 regulates the expression of genes encoding key elements of stomatal functioning and morphogenesis, such as a K+ channel protein, MYB60 transcription factor, and pectin methyl esterase. Consequently, ion homeostasis was disturbed in scap1 guard cells, and esterification of extracellular pectins was impaired so that the cell walls lining the pores did not mature normally. We conclude that SCAP1 regulates essential processes of stomatal guard cell maturation and functions as a key transcription factor regulating the final stages of guard cell differentiation.