This study focuses on the application of the ECOSSE-model to estimate greenhouse gas (GHG) emissions (CO2, N2O, and CH4) from an intensively managed grazed grassland located in Easter Bush, south-east Scotland (55° 52′ N, 03° 02′ W). The field has been under continuous permanent grassland management for about 20 years with a species composition of perennial ryegrass and white clover. The soil at the site is classified as mineral soil (i.e., sandy clay loam) with a pH of 5.1 and a clay fraction of 20–26%. The necessary field data on soil, climate, and management practices were collected and utilized as input for the model. Additionally, nine years of CO2 data and four years of N2O and CH4 data were gathered for model validation. The findings indicate that while the current version of the ECOSSE model may not be entirely reliable for estimating daily CO2 and N2O fluxes, it can still be a useful tool for predicting cumulative fluxes with a relative deviation of -36% and -3%, respectively. Notably, both observed and simulated CH4 emissions were consistently low and variable. The study highlights the importance of further developing and applying the ECOSSE model to estimate GHG emissions from permanent grasslands accurately.