Thiazide diuretics enhance renal Na⁺ excretion by blocking the Na⁺-Cl^– cotransporter (NCC), and mutations in NCC result in Gitelman syndrome. The mechanisms underlying the accompanying hypocalciuria and hypomagnesemia remain debated. Here, we show that enhanced passive Ca²⁺ transport in the proximal tubule rather than active Ca²⁺ transport in distal convolution explains thiazide-induced hypocalciuria. First, micropuncture experiments in mice demonstrated increased reabsorption of Na⁺ and Ca²⁺ in the proximal tubule during chronic hydrochlorothiazide (HCTZ) treatment, whereas Ca²⁺ reabsorption in distal convolution appeared unaffected. Second, HCTZ administration still induced hypocalciuria in transient receptor potential channel subfamily V, member 5–knockout (Trpv5-knockout) mice, in which active distal Ca²⁺ reabsorption is abolished due to inactivation of the epithelial Ca²⁺ channel Trpv5. Third, HCTZ upregulated the Na⁺/H⁺ exchanger, responsible for the majority of Na⁺ and, consequently, Ca²⁺ reabsorption in the proximal tubule, while the expression of proteins involved in active Ca²⁺ transport was unaltered. Fourth, experiments addressing the time-dependent effect of a single dose of HCTZ showed that the development of hypocalciuria parallels a compensatory increase in Na⁺ reabsorption secondary to an initial natriuresis. Hypomagnesemia developed during chronic HCTZ administration and in NCC-knockout mice, an animal model of Gitelman syndrome, accompanied by downregulation of the epithelial Mg²⁺ channel transient receptor potential channel subfamily M, member 6 (Trpm6). Thus, Trpm6 downregulation may represent a general mechanism involved in the pathogenesis of hypomagnesemia accompanying NCC inhibition or inactivation.