Background/Objectives: Contrast-induced nephropathy (CIN) is a well-documented complication following coronary angiography and percutaneous coronary intervention (PCI). This study aims to evaluate the predictors of CIN and its effect on outcomes in patients with acute coronary syndrome (ACS). Methods: A retrospective study included 1579 patients who underwent coronary angiography or PCI. Results: The overall incidence of in-hospital CIN was 6.8%, with the highest incidence in the group with eGFR <30 mL/min/1.73 m² at 21.6%. Non-radial vascular access was an independent predictor of CIN occurrence (OR = 2.06 [1.37–3.08]; p < 0.001). The risk of death within 30 days was influenced by history of stroke (OR = 4.94 [1.58–15.51]; p = 0.006), glucose level on admission (per 10-unit increase) (OR = 1.07 [1.04–1.1]; p < 0.001), occurrence of CIN (OR = 5.64 [2.49–12.79]; p < 0.001), and hemoglobin level (OR = 0.77 [0.65–0.92]; p = 0.003). The risk of death within 365 days was increased by age (OR = 1.05 [1.02–1.07]; p < 0.001), history of stroke (OR = 2.45 [1.02–5.89]; p = 0.046), glucose levels on admission (per 10-unit increase) (OR = 1.05 [1.03–1.08]; p < 0.001), occurrence of CIN (OR = 2.62 [1.42–4.84]; p = 0.002), and hemoglobin concentration (OR = 0.78 [0.7–0.88]; p = 0.003). An independent predictor of hospitalization for acute or exacerbation of chronic renal failure was baseline creatinine concentration (OR = 3.44 [2.4–4.93]; p < 0.001). Conclusions: The incidence of CIN is significant, particularly in patients with severe pre-existing renal impairment. Non-radial vascular access is an independent predictor of CIN. The occurrence of CIN is a strong independent predictor of both short-term and long-term mortality.