During plant epidermal development, many cell types are generated from protodermal cells, a process requiring complex co-ordination of cell division, growth, endoreduplication and the acquisition of differentiated cellular morphologies. Here we show that the Arabidopsis phytocalpain DEFECTIVE KERNEL 1 (DEK1) promotes the differentiated epidermal state. Plants with reduced DEK1 activity produce cotyledon epidermis with protodermal characteristics, despite showing normal growth and endoreduplication. Furthermore, in non-embryonic tissues (true leaves, sepals), DEK1 is required for epidermis differentiation maintenance. We show that the HD-ZIP IV family of epidermis-specific differentiation-promoting transcription factors are key, albeit indirect, targets of DEK1 activity. We propose a model in which DEK1 influences HD-ZIP IV gene expression, and thus epidermis differentiation, by promoting cell adhesion and communication in the epidermis.