Plant trichomes originate from epidermal cells.  In this work, we demonstrated that a homeodomain-leucine zipper (HD-Zip) gene, Gh_A06G1283 (GhHD-1A), was related to the leaf trichome trait in allotetraploid cotton and could be a candidate gene for the T₁ locus.  The ortholog of GhHD-1A  in the hairless accession Gossypium barbadense cv. Hai7124 was interrupted by a long terminal repeat (LTR) retrotransposon, while GhHD-1A worked well in the hairy accession Gossypium hirsutum acc. T586.  Sequence and phylogenetic analysis showed that GhHD-1A  belonged to the HD-Zip IV gene family, which mainly regulated epidermis hair development in plants.  Silencing of GhHD-1A  and its homoeologs GhHD-1D in allotetraploid T586 and Hai7124 could significantly reduce the density of leaf hairs and affect the expression levels of other genes related to leaf trichome formation.  Further analysis found that GhHD-1A  mainly regulated trichome initiation on the upper epidermal hairs of leaves in cotton, while the up-regulated expression of GhHD-1A  in different organs/tissues also altered epidermal trichome development.  This study not only helps to unravel the important roles of GhHD-1A  in regulating trichome initiation in cotton, but also provides a reference for exploring the different forms of trichome development in plants.