Epithelial ovarian cancer (EOC) is usually diagnosed at an advanced stage and has poor prognosis. Theranostic agents are the current trend in drug development, but are lacking in EOC. YKL40 is predominantly expressed and involved in tumorigenesis in EOC. In this study, we developed a companion theranostic agent targeting YKL40. We measured YKL40 expression levels in ascites using ELISA and correlated them with the clinical outcomes of patients with EOC. We developed radionuclide labeled In-111/Lu-177-DTPA-YKL40 neutralizing antibodies and investigated their radiochemical purity, SPECT/CT imaging, bio-distribution, and therapeutic responses in ovarian cancer xenograft mice. We demonstrated that YKL40 expression levels in ascites were significantly higher in EOC patients with serous histological type, high tumor grade, advanced stage, tumor recurrence, chemoresistance, and tumor-related death. The radiochemical purity of In-111/Lu-177-DTPA-YKL40 neutralizing antibodies reached more than 90% after 24 h of labeling. SPECT/CT imaging showed significant accumulation of In-111-DTPA-YKL40 and Lu-177-DTPA-YKL40 antibodies at the tumor site of ovarian cancer xenograft mice 24 h after administration. Lu-177-DTPA-YKL40 antibodies significantly inhibited tumor growth in ovarian cancer xenograft mice. Our study indicated that In-111/Lu-177-DTPA-YKL40 neutralizing antibodies could be potential companion theranostic agents for patients with EOC.