A fully biodegradable zwitterionic polymer and the corresponding conjugate with paclitaxel (PTX) were synthesized as promising biomaterials. Allyl-functionalized polylactide (PLA) was employed as the precursor of polymer backbones. UV-induced thiol-ene reaction was conducted to conjugate thiol-functionalized sulfobetaine (SB) with the PLA-based backbone. The resulting zwitterionic polymer did not exhibit considerable cytotoxicity. A polymer-drug conjugate was also obtained by thiol-ene reaction of both thiol-functionalized SB and PTX with allyl-functionalized PLA. The conjugate could readily form narrowly-dispersed nanoparticles in aqueous solutions with a volume-average hydrodynamic diameter (Dh,V) of 19.3 ± 0.2 nm. Such a polymer-drug conjugate-based drug delivery system showed full degradability, well-suppressed non-specific interaction with biomolecules, and sustained drug release. In vitro assessments also confirmed the significant anti-cancer efficacy of the conjugate. After 72 h incubation with PLA-SB/PTX containing 10 µg/mL of PTX, the cell viabilities of A549, MCF7, and PaCa-2 cells were as low as 20.0 ± 2.5%, 1.7 ± 1.7%, and 14.8 ± 0.9%, respectively. Both flow cytometry and confocal microscopy suggested that the conjugates could be easily uptaken by A549 cells before the major release of PTX moieties. Overall, this work elucidates promising potentials of biodegradable zwitterionic polymer-based materials in biomedical applications. Statement of Significance The applicability of FDA-approved biodegradable aliphatic polyesters has been significantly restricted because they are hydrophobic and lack functionalities. Recently zwitterionic polymers have emerged as promising hydrophilic biomaterials, but most of the reported zwitterionic polymers are non-biodegradable. This study reports a novel aliphatic polyester-based zwitterionic polymer and the corresponding polymer-drug conjugate. Their aliphatic polyester and zwitterionic components provide them with high enzymatic degradability and low nonspecific interactions with biomolecules, respectively. While the zwitterionic polymer did not show noticeable cytotoxicity, the corresponding polymer-anticancer drug conjugate exhibited acid-sensitive sustained drug release, remarkable effectiveness in killing cancer cells, as well as the ready cellular internalization. This work lays a foundation for the further development of synthetic biodegradable zwitterionic polymer-based materials which potentially may have broad and significant biomedical applications.