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• DOX/IpYR achieved high penetration and long-term retention simultaneously. • DOX/IpYR regulated the expression of immune factors and PD-L1. • DOX/IpYR suppressed tumour growth and metastases. Conventional intratumoural drug delivery systems often struggle to achieve both high permeability and long-term retention. Smaller nanoparticles (NPs) readily diffuse into deep tumour regions, but their rapid clearance results in lower accumulation at tumour sites. Furthermore, although immunostimulatory molecules have been introduced for the immunotherapy of tumours, it is crucial to develop drug nanovehicles with immunomodulatory functions. Here, a novel self-assembled amphiphilic peptide, IpYR, self-assembled into spherical NPs after drug loading, which were transformed into nanofibres (NFs) with a high aspect ratio under the induction of alkaline phosphatase (ALP). This drug-loading system effectively prolonged the residence time of the chemotherapeutic drug in tumour tissues and displayed enhanced aggregation at tumour sites in vivo. Furthermore, the transformable peptide NPs not only inhibited tumour growth but also suppressed tumour metastasis. Importantly, the peptide nanovehicles inhibited the proliferation of Treg cells, downregulated the expression of immunosuppressive factors and PD-L1, and simultaneously stimulated the expression of immunogenic factors, effectively alleviating the immunosuppressive microenvironment. In conclusion, this transformable peptide-based nanodelivery system is a concise and promising nanoplatform for maintaining potent drug enrichment and inhibiting immunosuppression in the tumour microenvironment. [ABSTRACT FROM AUTHOR]