BACKGROUND AND PURPOSE: TMEM16A Ca2+-gated Cl- channels mediate depolarisation of contractile vascular cells. The anthelmintic niclosamide was reported to modulate the TMEM16A channel, suggesting possible repurposing for vascular pharmacology. Here, we investigate the mechanism of TMEM16A modulation by niclosamide and explore its effect on the function of a range of vessel types. EXPERIMENTAL APPROACH: Patch-clamp electrophysiology, alongside genetically encoded systems to modulate plasmalemmal PIP2 content, was used to define the mechanism of action of niclosamide on the TMEM16A channel. Vascular contractility was investigated using isometric tension recordings of isolated rat arteries and differential interference contrast imaging of capillary diameter in rat brain slices. KEY RESULTS: In low intracellular free Ca2+ concentrations ([Ca2+]i), clinically relevant niclosamide concentrations inhibited or enhanced heterologous TMEM16A currents at positive or negative membrane potentials (Vm), respectively. In saturating [Ca2+]i, niclosamide inhibited the channel at each Vm tested, independent of plasmalemmal PIP2 levels. Niclosamide caused a transient contraction of isolated aortae and mesenteric and pulmonary arteries but dampened responses to phenylephrine, a Gq protein-coupled receptor (GqPCR) agonist. Niclosamide reduced brain cortical pericyte constriction evoked by endothelin-1. Unlike Ani9, a selective TMEM16A inhibitor, niclosamide reduced arterial response to elevated extracellular K+. Niclosamide also inhibited heterologous and native voltage-gated Ca2+ (CaV) currents in smooth muscle cells. CONCLUSION AND IMPLICATIONS: Niclosamide dampened vascular responses to GqPCR stimulation due to concomitant modulation of TMEM16A and CaV channels. Elucidating the molecular pharmacology of niclosamide supports its potential use in disorders of altered vessel tone including stroke, hypertension and vascular dementia.