Background: Serum C-reactive protein (CRP) is an established biomarker for acute inflammation and has been identified as a prognostic indicator for hepatocellular carcinoma (HCC). However, the significance of the serum CRP level, specifically in HCC patients treated with lenvatinib, remains unclear. Methods: We retrospectively analyzed 125 HCC patients who received lenvatinib treatment at six centers. Clinical characteristics were assessed to identify clinical associations between serum CRP and HCC prognosis. Results: The median overall serum CRP level was 0.29 mg/dL. The cohort was divided into two groups: the low-CRP group with a serum CRP < 0.5 mg/dL and the high-CRP group with a serum CRP ≥ 0.5 mg/dL. The low-CRP group exhibited significantly longer overall survival (OS) than the high-CRP group (22.9 vs. 7.8 months, p < 0.001). No significant difference was observed for progression-free survival (PFS) between the high- and low-CRP groups (9.8 vs. 8.4 months, p = 0.411), while time-to-treatment failure (TTF) was significantly longer in the low-CRP group (8.5 vs. 4.4 months, p = 0.007). The discontinuation rate due to poor performance status was significantly higher in the high-CRP group (p < 0.001). Conclusion: A baseline serum CRP level exceeding 0.5 mg/dL was identified as an unfavorable prognostic factor in HCC patients receiving lenvatinib treatment.