Herein, we present the rational design and facile synthesis of high‐refractive‐index copolymers with good optical transparency and tunable thermal stability using aromatic heterocycle‐based vinyl sulfides via conventional radical copolymerization. 2‐Thiazolylvinylsulfide (2TVS), 1,3,4‐thiadiazolylvinylsulfide (1,3,4‐TVS), 3‐methylimidazolylvinylsulfide (3MIVS), and 5‐methlthio‐1,3,4‐thiadiazolylvinylsulfide (MeSTVS) were selected to enhance the refractive index, and N‐vinylphthalimide (NVPI) and N‐phenylmaleimide (NPMI) were chosen as comonomers to achieve improved transparency and thermal stability while maintaining the refractive index. The sulfur, ‐C = N‐, and imide contents in the copolymers were varied by adjusting the monomer structures and their compositions, and their effects on the refractive index, Abbe number, and optical and thermal properties were investigated. The MeTVS monomer in poly(MeSTVS) and poly(MeSTVS‐co‐NVPI) contained three sulfur atoms and two ‐C = N‐ units; consequently, these polymers exhibited excellent refractive indices in the 1.7139–1.6567 range at 589 nm, Abbe numbers between 19.1–29.1, good transparency (>90% at 400 nm), and tunable thermal properties (Tg = 49–138 °C, Td10 = 244–268 °C). Optically transparent poly(1,3,4‐TVS‐co‐NVPI) films (> 95% at 400 nm) with improved thermal stability (Tg = 89–175 °C, Td10 = 251–301 °C) were obtained, while maintaining a relatively high refractive index (1.6662–1.6752).This article is protected by copyright. All rights reserved