Heparin-like sulfated polysaccharides are potential drug candidates owing to their ability to interact with angiogenic factors and inhibit angiogenesis, tumor growth, and metastasis. This study aimed to improve the delivery of heparin-like anticancer polysaccharides for accumulation at the tumor site. We designed a nanocarrier system using protamine attached to polyethylene glycol (PEG) and evaluated the stability, tumor targeting, and tumor growth inhibition of the nanocarrier loaded with heparin derivatives. When mixed with various polyanionic heparin derivatives, the polycationic PEG–protamine formed stable self-assembled nanocomplexes via ionic interactions, with flexible PEG chains located on the outside. Among the complexes, a nanocomplex loaded with a low-molecular-weight heparin–suramin conjugate (LHsura) had the most suitable average size (101.9 nm) for the enhanced permeability and retention effect and allowed accumulation of LHsura at the tumor site for up to 48 h. In a tumor-bearing mouse model, the PEG–protamine and LHsura nanocomplex (10 mg/kg/3 days, intravenously), which could be extravasated through the tumor vasculature, significantly inhibited tumor growth, more than LHsura alone did. Overall, the self-assembled nanocomplexation of PEG–protamine and LHsura helped control the release and extravasation of LHsura, which resulted in an antitumor effect on the target tumor cells.