Leaf shape is spectacularly diverse. As the primary source of photo-assimilate in major crops, understanding the evolutionary and environmentally induced changes in leaf morphology are critical to improving agricultural productivity. The role of leaf shape in cotton domestication is unique, as breeders have purposefully selected for entire and lobed leaf morphs resulting from a single locus, okra (L-D1). The okra locus is not only of agricultural importance in cotton (Gossypium hirsutum L.), but through pioneering chimeric and morphometric studies it has contributed to fundamental knowledge about leaf development. Here we show that the major leaf shapes of cotton at the L-D1 locus are controlled by a HD-Zip transcription factor most similar to Late Meristem Identity1 (LMI1) gene. The classical okra leaf shape gene has 133-bp tandem duplication in the promoter, correlated with elevated expression, while an 8-bp deletion in the third exon of the presumed wild-type normal leaf causes a frame-shifted and truncated coding sequence. Virus-induced gene silencing (VIGS) of this LMI1-like gene in an okra variety was sufficient to induce normal leaf formation. An intermediate leaf shape allele, sub-okra, lacks both the promoter duplication and the exonic deletion. Our results indicate that sub-okra is the ancestral leaf shape of tetraploid cotton and normal is a derived mutant allele that came to predominate and define the leaf shape of cultivated cotton.