A self‐stabilized supramolecular assembly (SSA) from the PEGylated dendritic peptide conjugate is presented, which builds on the concentration‐dependent supramolecular self‐assembly that demonstrates ultrahigh colloidal stability and enhanced tumor retention to boost its photodynamic therapeutic efficiency. This study reveals the supramolecular self‐assembly mechanism of SSAs and further expands the advanced functions of dendritic peptides in the field of cancer therapy. Supramolecular self‐assemblies of dendritic peptides with well‐organized nanostructures have great potential as multifunctional biomaterials, yet the complex self‐assembly mechanism hampers their wide exploration. Herein, a self‐stabilized supramolecular assembly (SSA) constructed from a PEGylated dendritic peptide conjugate (PEG‐dendritic peptide‐pyropheophorbide a, PDPP), for augmenting tumor retention and therapy, is reported. The supramolecular self‐assembly process of PDPP is concentration‐dependent with multiple morphologies. By tailoring the concentration of PDPP, the supramolecular self‐assembly is driven by noncovalent interactions to form a variety of SSAs (unimolecular micelles, oligomeric aggregates, and multi‐aggregates) with different sizes from nanometer to micrometer. SSAs at 100 nm with a spherical shape possess extremely high stability to prolong blood circulation about 4.8‐fold higher than pyropheophorbide a (Ppa), and enhance tumor retention about eight‐fold higher than Ppa on day 5 after injection, which leads to greatly boosting the in vivo photodynamic therapeutic efficiency. RNA‐seq demonstrates that these effects of SSAs are related to the inhibition of MET‐PI3K‐Akt pathway. Overall, the supramolecular self‐assembly mechanism for the synthetic PEGylated dendritic peptide conjugate sheds new light on the development of supramolecular assemblies for tumor therapy.