Motion sickness (MS) is a common phenomenon in life and nearly all individuals will experience MS at least once in their lives. Diphenidol hydrochloride (DPN) is widely used in China for the pharmacotherapy of MS at the dosages of 25 mg every 6-8 h as needed. The conventional DPN tablets may present inconvenience for individuals embarking on extended sea voyages, as they require multiple daily doses. The aim of this study was to develop and assess in vitro a novel tablet-in-tablet (TIT) formulation of DPN, which serves as a single-dose unit designed to facilitate rapid and extended drug release. Fourier transform infrared (FTIR) and differential scanning calorimetry (DSC) analyses were employed to investigate the possible interactions between DPN and adjuvants in the pre-formulation study. Direct compression (DC) technique was employed to fabricate the tablet, employing microcrystalline cellulose (MCC) and polyethylene oxide (PEO) as release modifying agents. The study investigated the influence of MCC and PEO, each at various weights, on the in vitro drug release and swelling index of core-tablets using a response surface method (RSM) of Miscellaneous (3-level Factorial) design. It demonstrated that an increase in PEO content enhanced drug release, whereas the inclusion of MCC exerted a suppressive influence on release. The core tablet was press-coated with an outer instant-release shell, which composed of MCC and cross-linked sodium carboxymethylcellulose (CMC) in three distinct weight. Both the core-tablet and TIT exhibited desirable physicochemical properties. Optimized formulation of the core-tablet (PEO 100 mg, MCC 70 mg) achieved a 90% of drug release in vitro after 12 h and a swelling index of 52%. The final TIT formulation (Core tablet: PEO 100 mg, MCC 70 mg; Outer shell: PEO 70 mg, MCC 300 mg) was finally obtained with reduced excipient content and shorter disintegration time. The developed TIT formulation of DPN may offer a promising approach for improving the convenience and efficacy of MS treatment by providing both immediate and extended drug release from a single dosage form.© 2025. The Author(s).