Aim and objective: The aim of the research was to evaluate the histocompatibility of thermopolymerized (TP) novel denture copolymer containing 3,9-dimethylene-1,5,7,11-tetraoxaspiro[5,5]undecane (DMTOSU) comonomer in rats’ palatine tissue. Materials and methods: The rats were randomly categorized into four groups (n = 6 per group). GCW: Denture base appliance (DBA) fabricated by short polymerization cycle in a water bath without DMTOSU; GTW: DBA with 20 wt.% DMTOSU polymerized at 70°C for 2 hours followed by short polymerization cycle in a water bath; GTA: DBA with 20 wt.% DMTOSU polymerized at 60°C for 45 minutes followed by 130°C for 20 minutes in an autoclave; and Group NC (negative control): rats with no DBA. The rats were euthanized after 2 weeks and the palatal tissues were subjected to histological examination. Epithelial thickness (ET), connective tissue thickness (CT), and keratin layer thickness (KT) were measured. Results: GCW exhibited greater ET, CT, and KT than the other groups. The ET and KT of GTA were significantly lesser than GTW. Multiple comparisons exhibited significant differences between the groups, except for GTW and GTA concerning the CT. Conclusion: The novel denture copolymer containing 20 wt.% of DMTOSU comonomer is histocompatible with rats’ palatine tissue. Clinical significance: As DMTOSU is a double-ring-opening antishrinking oxaspiro monomer, its incorporation in TP-DBR would result in dimensionally accurate and stable dentures without endangering the biocompatibility in the prospective years. © 2022 Elsevier B.V., All rights reserved.