Self-assembly of site-selective protein-polymer conjugates into stimuli-responsive micelles is interesting owing to their potential biomedical applications, ranging from molecular imaging to drug delivery, but remains a significant challenge. Herein we report a method of site-selective in situ growth-induced self-assembly (SIGS) to synthesize site-specific human serum albumin-poly(2-(diisopropylamino)ethyl methacrylate) (HSA-PDPA) conjugates that can in situ self-assemble into pH-responsive micelles with tunable morphologies. Indocyanine green (ICG) was selectively loaded into the core of sphere-like HSA-PDPA micelles to form pH-responsive fluorescence nanoprobes. The nanoprobes rapidly dissociated into protonated individual unimers at a transition pH of around 6.5, that is the extracellular pH of tumors, which resulted in a sharp fluorescence increase and markedly enhanced cellular uptake. In a tumor-bearing mouse model, they exhibited greatly enhanced tumor fluorescence imaging as compared to ICG alone and pH-nonresponsive nanoprobes. These findings suggest that pH-responsive and site-selective protein-polymer conjugate micelles synthesized by SIGS are promising as a new class of tumor microenvironment-responsive nanocarriers for enhanced tumor imaging and therapy.