Disturbances in calcium-phosphate homeostasis are common after kidney transplantation. We aimed to assess the relationship between deregulations in plasma calcium and phosphate over time and mortality and death-censored graft failure (DCGF). In this prospective cohort study, we included kidney transplant recipients with ≥2 plasma calcium and phosphate measurements. Data were analyzed using time-updated Cox regression analyses adjusted for potential confounders including time-updated kidney function. We included 2769 patients (mean age 47 ± 14 years, 42.3% female) with 138 496 plasma calcium and phosphate levels (median [IQR] 43 [31–61] measurements per patient). During follow-up of 16.3 [8.7–25.2] years, 17.2% developed DCGF and 7.9% died. Posttransplant hypercalcemia was associated with an increased risk of mortality (1.63 [1.31–2.00], p < 0.0001), but not with DCGF. Hyperphosphatemia was associated with both DCGF (2.59 [2.05–3.27], p <.0001) and mortality (3.14 [2.58–3.82], p <.0001). Only the association between hypercalcemia and mortality remained significant in sensitivity analyses censored by a simultaneous eGFR <45 mL/min/1.73 m 2. Hypocalcemia and hypophosphatemia were not consistently associated with either outcome. Posttransplant hypercalcemia, even in the presence of preserved kidney function, was associated with an increased mortality risk. Associations of hyperphosphatemia with DCGF and mortality may be driven by eGFR.