The immunomodulatory effects of many therapeutic agents are significantly challenged by their insufficient delivery efficiency and short retention time in tumors. Regarding the distinctively upregulated fibronectin (FN1) and tenascin C (TNC) in tumor stroma, herein a protease-activated FN1 and/or TNC binding peptide (FTF) is designed and an extracellular matrix (ECM)-trapped bioinspired lipoprotein (BL) (FTF-BL-CP) is proposed that can be preferentially captured by the TNC and/or FN1 for tumor retention, and then be responsively dissociated from the matrix to potentiate the antitumor immunity. The FTF-BL-CP treatment produces a 6.96-, 9.24-, 6.72-, 7.32-, and 6.73-fold increase of CD3+CD8+ T cells and their interferon-γ-, granzyme B-, perforin-, and Ki67-expressing subtypes versus the negative control, thereby profoundly eliciting the antitumor immunity. In orthotopic and lung metastatic breast cancer models, FTF-BL-CP produces notable therapeutic benefits of retarding tumor growth, extending survivals, and inhibiting lung metastasis. Therefore, this ECM-trapping strategy provides an encouraging possibility of prolonging tumor retention to potentiate the antitumor immunity for anticancer immunotherapy.