The 1,4-dihydropyridines, drugs with well-established bioavailability and toxicity profiles, have proven efficacy in treating human hypertension, peripheral vascular disorders, and coronary artery disease. Every 1,4-dihydropyridine in clinical use blocks L-type voltage-gated calcium channels. We now report our development, using selective optimization of a side activity (SOSA), of a class of 1,4-dihydropyridines that selectively and potently inhibit the intermediate-conductance calcium-activated K+ channel KCa3.1, a validated therapeutic target for diseases affecting many organ systems. One of these 1,4-dihydropyridines, DHP-103, blocked KCa3.1 with an IC50 of 6 nM and exhibited exquisite selectivity over calcium channels and a panel of >100 additional molecular targets. Using high-resolution structure determination by cryogenic electron microscopy together with mutagenesis and electrophysiology, we delineated the drug binding pocket for DHP-103 within the water-filled central cavity of the KCa3.1 channel pore, where bound drug directly impedes ion permeation. DHP-103 inhibited gain-of-function mutant KCa3.1 channels that cause hereditary xerocytosis, suggesting its potential use as a therapeutic for this hemolytic anemia. In a rat model of acute ischemic stroke, the second leading cause of death worldwide, DHP-103 administered 12 h postischemic insult in proof-of-concept studies reduced infarct volume, improved balance beam performance (measure of proprioception) and decreased numbers of activated microglia in infarcted areas. KCa3.1-selective 1,4-dihydropyridines hold promise for the many diseases for which KCa3.1 has been experimentally confirmed as a therapeutic target.