Hyperbranched copolymers with different degrees of branching (DBs), poly(3,4-dihydroxycinnamic acid)-co-poly(4-hydroxycinnamic acid) (PCA), were prepared by polycondensation. Amphiphilic PCA–DTT copolymers were prepared by grafting dithiothreitiol (DTT) into PCA by the Michael addition. PCA–DTT nanoparticles were self-assembled by additional water into a DMSO solution of PCA–DTT. The diameter and photoresponsivity of the PCA–DTT nanoparticles were influenced by the DB, and they increased with increasing the DB. Bovine serum albumin (BSA) as a model protein was successfully encapsulated into the PCA–DTT nanoparticles, and the release behavior of BSA was affected by the DB in PBS. These biodegradable and photoresponsive nanoparticles would be useful as functional carriers for drug delivery systems.