(1) Background: This observational study aimed to verify the association between serum potassium levels and hospitalization days in patients with chronic kidney disease in a follow up of nine months. (2) Methods: Patients with chronic kidney disease were divided into group A (180 patients, potassium ≤ 5.1 mEq/L) and B (90 patients, potassium > 5.1 mEq/L). Student's t-test, Mann-Whitney test, Pearson's Chi-Square test, Pearson/Spearman's correlation test and linear regression test were performed in the entire sample and in stage-G4/5 subsample. (3) Results: Groups A and B differed for estimated glomerular filtration rate (eGFR) (34.89 (IQR, 16.24-57.98) vs. 19.8 (IQR, 10.50-32.50) mL/min/1.73 m2; p < 0.0001), hemoglobin (11.64 ± 2.20 vs. 10.97 ± 2.19 g/dL, p = 0.048), sum of hospitalization days (8 (IQR, 6-10) vs. 11 (IQR, 7-15) days; p < 0.0001) and use of angiotensin II receptor blockers (40.2% vs. 53.3%; p = 0.010). Considering patients with eGFR 6-30 mL/min/1.73 m2, differences in the sum of hospitalization days were confirmed. Multivariable regression analysis showed that hyperkalemia is an independent risk factor of increased hospital length. In stage G4-G5, regression analysis showed that hyperkalemia is the only independent risk factor (β = 2.93, 95% confidence interval, 0.077-5.794, p = 0.044). (4) Conclusions: We observed significantly greater odds of increased length of hospital stay among patients with higher potassium, mostly in stages G4-G5 chronic kidney disease.