Here, we present the synthesis, physicochemical, and preliminary biological characterization of micellar polymer-betulinic acid (BA) conjugates based on N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer carriers, enabling the controlled release of cytotoxic BA derivatives in solid tumors or tumor cells. Various HPMA copolymer conjugates differing in the structure of the spacer between the drug and the carrier were synthesized, all designed for pH-triggered drug release in tumor tissue or tumor cells. The high molecular weight of the micellar conjugates should improve the uptake of the drug in solid tumors due to the Enhanced permeability and retention (EPR) effect. Nevertheless, only the conjugate containing BA with methylated carboxyl groups enabled pH-dependent controlled release in vitro. Moreover, drug release led to the disassembly of the micellar structure, which facilitated elimination of the water-soluble HPMA copolymer carrier from the body by renal filtration. The methylated BA derivative and its polymer conjugate exhibited high cytostatic activity against DLD-1, HT-29, and HeLa carcinoma cell lines and enhanced tumor accumulation in HT-29 xenograft in mice.