Here, we report a straightforward synthetic procedure for the preparation of metal-containing polymer microspheres with combined magnetic resonance imaging (MRI) and controlled anti-cancer drug release functionality confined in a single particle entity. Previously, we have reported a synthetic strategy to achieve gadolinium (Gd^III)-containing polymer nanoparticles with outstanding MRI capabilities. In this study, we now address novel dual-functional theranostic agents based on poly(gadolinium methacrylate-co-methacrylic acid) (poly(Gd(MAA)₃-co-MAA)) copolymer microspheres, which could be additionally loaded with the anti-cancer drug doxorubicin (DOX) through a highly efficient and reversible hydrophilic interaction. Both diagnostic and therapeutic features were thoroughly investigated, and experimental results demonstrated that the polymer microspheres had an exceptionally high drug loading efficiency of about 86 wt% at pH 7.4, and the corresponding anti-cancer drug-loaded microspheres exhibited excellent controlled drug releasing properties under physiological conditions of tumor cells (pH < 7), accompanied by a remarkable induced apoptosis for SKOV-3 cancer cells. Moreover, microspheres investigated herein possessed a sufficiently low cytotoxicity to normal HEK-293T cells and an outstanding longitudinal relaxivity of γ₁ = 10.639 mM⁻¹ s⁻¹ and could be successfully applied in in vivo MRI experiments. The herein described poly(Gd(MAA)₃-co-MAA) microspheres provide access to versatile nanotherapeutic systems capable of multiple activities including diagnostic imaging, therapeutic intervention and therapy response monitoring.