European Centre for Medium-Range Weather Forecasts Reanalysis 5 (ERA5) is a state-of-the-art reanalysis dataset and has been widely used in climate change analysis and land surface process simulations. However, few assessments have been conducted across the entire ocean for this dataset till now. The motivation of this study is to utilize the in-situ observations and remotely sensed sea surface temperature (SST) products to propose a new approach to evaluate the precision of ERA5 air temperature over the whole ocean. First, a stacked multilayer perceptron regressor model was employed to map global marine air temperature (MAT) by incorporating in-situ records into moderate-resolution imaging spectroradiometer (MODIS) SST products. Second, a monthly MAT dataset with a spatial resolution of 0.05° × 0.05° was developed and validated for the assessment of ERA5 air temperature particularly in the regions where only satellite observations are available. It demonstrates that the quality of estimated MAT (RMSE = 1.62 °C, Bias = 0.03 °C) is evidently high in 80.3% of the observation stations, which is much better than that of averaged SST (RMSE = 2.29 °C, Bias = 1.14 °C). Thus, the estimated MAT is credible enough to be employed to assess ERA5 air temperature both at the grid and regional scale. Finally, the ERA5 air temperature was compared and validated with the estimated MAT temporally and spatially. The further study suggested that estimated MAT and ERA5 air temperature basically maintain consistency in expressing persistent warming, particularly in the western Pacific, western Atlantic, and northern Indian Ocean. In addition, the trend of ERA5 air temperature during 50°S-50°N reaches 0.016 ± 0.003°C/yr, which is similar to that of estimated MAT (0.017 ± 0.002 °C/yr) during 2003-2021.