<p>In this study, a one-dimensional ternary photonic crystal (1D TPC) with a defect layer is proposed and theoretically demonstrated as a sensor for detecting seawater salinity. Seawater is introduced as a defect layer inserted at the middle of 1D TPC, breaking the structural symmetry and thereby generating a defect mode within the photonic band gap. By utilizing the transfer matrix method (TMM), theoretical results indicate that the peak wavelength of the defect mode in the band gap is linearly proportional to salinity, namely, the resonant peak is red-shift (blue-shift) with increasing (decreasing) the salinity. Consequently, seawater salinity is accurately determined by tracking the peak wavelength of the defect mode. Under conditions of a defect layer thickness of 850&#xa0;nm, a temperature of 25&#xa0;°C, and normal incidence, an ultrahigh sensitivity of 2.687ⅹ10⁵ nm/RIU is achieved when the salinity alternates between 35‰ and 25‰. Additional performance parameters such as a quality factor (<i>Q</i>) of 1.135ⅹ10⁸, a figure of merit (<i>FOM</i>) of 1.675ⅹ10¹⁰ RIU^− 1, and a limit of detection (<i>LOD</i>) of 2.985ⅹ10^− 12 RIU are achieved at 35‰ salinity. Under oblique incidence, the defect mode induced by the seawater defect layer shifts to shorter wavelength as the incident angle increases. While the incident angle varies from 0⁰ to 80⁰, the Q factor of the defect mode remains on the order of 10⁸ for the TE polarized mode. In contrast, the Q value gradually decreases with increasing incident angle for the TM polarized mode. This research demonstrates that the 1D TPC based sensor for salinity detection offers significant advantages for advancing ocean engineering development.</p>