The putative secretory peptides epidermal patterning factor 1 (EPF1) and EPF2 act as negative regulators of stomatal clustering and density early in Arabidopsis leaf development. Here, we investigated whether the related peptide gene epidermal patterning factor-like 9 (EPFL9), which is coexpressed with EPF1 and stomatal density and distribution 1 (SDD1), also plays a role in controlling stomatal development. Plants manipulated to constitutively overexpress EPFL9 showed increased stomatal density and clustering, and those manipulated to have reduced EPFL9 expression showed reduced stomatal density with no clustering, confirming that EPFL9 is a regulator of stomatal development. Genetic analysis was consistent with EPFL9 acting independently of EPF1 to control stomatal clustering, independently of EPF2 to regulate stomatal density, and independently of SDD1 to control both stomatal clustering and density. These findings demonstrate that at least three secretory peptides independently regulate stomatal development. Surprisingly, EPFL9 acts to increase, rather than decrease, stomatal density and clustering. However, in common with EPF1 and EPF2, EPFL9 is unlikely to be a substrate for proteolysis by SDD1.