The concentration of free cytosolic Ca²⁺ and the voltage across the plasma membrane are major determinants of cell function. Ca²⁺-permeable non-selective cationic channels are known to regulate these parameters but understanding of these channels remains inadequate. Here we focus on Transient Receptor Potential Canonical 4 and 5 proteins (TRPC4 and TRPC5) which assemble as homomers or heteromerize with TRPC1 to form Ca²⁺-permeable non-selective cationic channels in many mammalian cell types. Multiple roles have been suggested including in epilepsy, innate fear, pain and cardiac remodeling but limitations in tools to probe these channels have restricted progress. A key question is whether we can overcome these limitations and develop tools which are high-quality, reliable, easy to use and readily accessible for all investigators. Here, through chemical synthesis and studies of native and over-expressed channels by Ca²⁺ and patch-clamp assays, we describe compound 31 (C31), a remarkable small-molecule inhibitor of TRPC1/4/5 channels. Its potency ranged from 9 to 1300 pM, depending on the TRPC1/4/5 subtype and activation mechanism. Other channel types investigated were unaffected, including TRPC3, TRPC6, TRPV1, TRPV4, TRPA1, TRPM2, TRPM8 and store-operated Ca²⁺ entry mediated by Orai1. These findings suggest identification of an important experimental tool compound which has much higher potency for inhibiting TRPC1/4/5 channels than previously reported agents, impressive specificity, and graded subtype selectivity within the TRPC1/4/5 channel family. The compound should greatly facilitate future studies of these ion channels. We suggest naming this TRPC1/4/5-inhibitory compound Pico145.