In this study, a novel pyrazole–carbodithioate-based chain transfer agent (CTA) featuring an aldehyde group (CTA-CHO) was designed and synthesized for RAFT polymerization. The obtained CTA-CHO was employed for the RAFT polymerization of styrene to afford well-defined polystyrenes bearing an aldehyde at their chain ends with low Ð values (~1.1). In addition, the reactivity of the aldehyde moiety at the end of the chain was precisely evaluated, while the Passerini three-component reaction was successfully performed on the aldehyde group. In this study, a novel pyrazole–carbodithioate-based chain transfer agent (CTA) featuring an aldehyde group (CTA-CHO) was designed and synthesized. The newly synthesized CTA-CHO was used for the RAFT polymerization of styrene to afford well-defined polystyrenes end-functionalized with an aldehyde unit (PSt-end-CHO). The obtained PSt-end-CHOs feature low Ð values (~1.1), suggesting a living nature of the polymerization. Furthermore, the Passerini three-component reaction was successfully performed on the PSt-end-CHOs.