L1 cell adhesion molecule (L1CAM) is overexpressed in various solid tumors, and its overexpression is linked to increased invasion, metastasis, angiogenesis, cancer stemness, and therapy resistance. However, the mechanisms of L1CAM-mediated carcinogenesis and radioresistance, particularly in high-grade serous ovarian carcinoma (HGSOC), the most common and lethal type of ovarian cancer, remain insufficiently understood. Anti-L1CAM radioimmunotherapy (RIT) using the radioisotopes Terbium-161 (Tb-161) and Lutetium-177 (Lu-177) has shown that Tb-161 is more cytotoxic than Lu-177, due to its distinct radioactive properties. Moreover, we identified a population of L1CAM+/CD133+ cells as cancer stem cells (CSCs) in ovarian cancer and demonstrated L1CAM's association with radioresistance. Understanding L1CAM-regulated downstream signaling and cellular responses to anti-L1CAM RIT with Tb-161, compared to Lu-177, is now critical. To address this, we established CRISPR-Cas9-mediated L1CAM knock-out (ΔL1CAM) in human ovarian cancer cells (OVCAR8) alongside a control cell line with a knockout of the nonessential gene AAVS1 (ΔAAVS1). In this study, quantitative phosphoproteomics, coupled with matching proteomics, revealed (i) L1CAM-dependent signaling pathways and biological processes by comparing ΔL1CAM with wild-type cells, and (ii) cellular responses to anti-L1CAM RIT with Tb-161 and Lu-177 in L1CAM-expressing ovarian cancer cells.