This study utilized a high-resolution, three-dimensional hydrodynamic model with improved model evaluation to investigate seasonal variations in key hydrographic conditions, including sea level, water temperature, salinity, current speed, and circulation in the Gulf of Thailand (GoT), as well as its interaction with the South China Sea (SCS). The analysis focuses on a climatological year calculated from a 15-year average for 2006–2020, which is categorized into four seasons: northeast monsoon, the first inter-monsoon, southwest monsoon, and the second inter-monsoon. Evaluation of model performance, based on observational data with temporal resolutions ranging from 30 min to monthly average with a duration from 10 months to 5 years, demonstrated good accuracy through high coefficients of determination and low root mean square errors. Results clearly depicted seasonal variability in hydrographic properties, characterized by alternating patterns of high and low sea level, high and low water temperatures, saline and fresh water, along with a persistent anticyclonic gyre in the central area of GoT and a smaller anticyclonic gyre in the southern area. Seasonal exchange flows between the SCS and the GoT were also evident, with the strongest outflow in northeast monsoon and the weakest in the second inter-monsoon.