The management of diabetic macular edema (DME), a condition that leads to an irreversible and severe visual impairment, remains a substantial challenge worldwide. In this study, we developed a bioadhesive nanoplatform with enhanced drug penetration to explore alternative treatment modalities for DME. This nanoparticulate formulation consisted of a sequence of an amphiphilic phenylboronic acid-based block and random glycopolymer with a high loading capacity for dexamethasone (DEX) of up to 20% and sustained drug release at a clinically relevant dose. The bioadhesive nanocarriers penetrated the sclera and choroid and were distributed in the retina under the action of phenylboronic acid to further promote drug permeation and retention in target lesions. The pathological analysis, electroretinography examination and immunofluorescence staining revealed that the nanoformulation of DEX much more markedly reduced the symptoms of and inflammation associated with DME than the drug alone, without affecting the function of the retina. Bioadhesive drug delivery systems are expected to be a feasible approach to treat DME and other fundus diseases.