Objective: Low-potassium diets are recommended to reduce serum potassium (Sk) and prevent complications of chronic kidney disease (CKD), but evidence underpinning this recommendation has not been systematically reviewed and synthesized. We conducted a systematic review comparing change in Sk, CKD progression, and mortality between those on a low-potassium versus unrestricted potassium diet. Methods: We searched Medline, AMED, PsycINFO, CINAHL, Cochrane Central Register of Controlled Trials, and Clinicaltrials.org from inception to 3 April 2018. We included randomized and observational studies that compared these outcomes in adults with CKD who ate a restricted versus unrestricted amount of dietary potassium. We pooled mean change in Sk and adjusted hazard ratios of disease progression and mortality using random-effects meta-analyses. Results: We identified 5,563 articles, of which seven studies (3,489 participants) met our inclusion criteria. We found very low-quality evidence that restricted (1,295 mg/d) versus unrestricted (1,570 mg/d) dietary potassium lowered Sk by −0.22 mEq/L (95% confidence interval [CI]: −0.33, −0.10; I2 = 0%). Higher (4,558 mg/d) versus lower (1,725 mg/d) dietary potassium was not significantly associated with disease progression (hazard ratio [HR]: 1.14, 95% CI: [0.77, 1.70] I² = 57%). Higher (4,414 mg/d) compared with lower (1,670 mg/d) dietary potassium intake was not significantly associated with reduced mortality risk (HR: 0.80, 95% CI: [0.46, 1.41] I² = 78%). Conclusions: Very-low-quality evidence supports consensus that dietary potassium restriction reduces Sk in normokalemia but whether this is associated with risk of death in those with CKD is uncertain. High-quality randomized controlled trials are needed.