<b><i>Background/Aims:</i></b> Dietary potassium (K⁺) has beneficial effects on blood pressure and cardiovascular (CV) outcomes. Recently, several epidemiological studies have revealed an association between urinary K⁺ excretion (as proxy for dietary intake) and better renal outcomes in subjects with chronic kidney disease (CKD). To address causality, we designed the “K⁺ in CKD” study. <b><i>Methods:</i></b> The K⁺ in CKD study is a multicenter, randomized, double blind, placebo-controlled clinical trial aiming to include 399 patients with hypertension, CKD stage 3b or 4 (estimated glomerular filtration rate [eGFR] 15–44 mL/min/1.73 m²), and an average eGFR decline &#x3e; 2 mL/min/1.73 m²/year. As safety measure, all included subjects will start with a 2-week open-label phase of 40 mmol potassium chloride daily. Patients who do not subsequently develop hyperkalemia (defined as serum K⁺ &#x3e;5.5 mmol/L) will be randomized to receive potassium chloride, potassium citrate (both K⁺ 40 mmol/day), or placebo for 2 years. The primary end point is the difference in eGFR after 2 years of treatment. Secondary end points include other renal outcomes (&#x3e; 30% decrease in eGFR, doubling of serum creatinine, end-stage renal disease, albuminuria), ambulatory blood pressure, CV events, all-cause mortality, and incidence of hyperkalemia. Several measurements will be performed to analyze the effects of potassium supplementation, including body composition monitoring, pulse wave velocity, plasma renin and aldosterone concentrations, urinary ammonium, and intracellular K⁺ concentrations. <b><i>Conclusion:</i></b> The K⁺ in CKD study will demonstrate if K⁺ sup­plementation has a renoprotective effect in progressive CKD, and whether alkali therapy has additional beneficial effects.