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In surgical oncology, precise intraoperative delineation of malignant tumors for complete yet non-excessive resection has been challenging. Near-infrared II (NIR-II, 1,000 to 3,000 nm) imaging enables deep-tissue, high-contrast visualization, but clinical translation is limited by the lack of robust probes. Cytalux, the first clinically approved NIR-I (700 to 1,000 nm) probe for folate receptor (FR)-positive cancers, is limited in shallow imaging depth and low tumor to normal tissue contrast. Here, we present "Octopus" (OCTP), a modular multi-arm PEG-based NIR-II probe targeting FR capable of rapid and sustained tumor accumulation upon systemic administration to afford superior tumor-to-background ratios and precise margin visualization by NIR-II imaging, significantly outperforming Cytalux in mouse models. Importantly, we uncovered that NIR-II imaging in the >1,300 nm emission range under NIR-I excitation completely suppressed tissue autofluorescence, allowing unambiguous molecular imaging of residual cancerous cells at tumor margins for resection. Pharmacokinetic and toxicity studies demonstrate rapid clearance, minimal off-target accumulation, and excellent biocompatibility. OCTP enables accurate, background-free >1,300 nm fluorescence-guided tumor surgery, promising as the next-generation NIR-II molecular imaging of cancer.