A physical method was developed to retrieve the land surface temperature (LST) from infrared atmospheric sounding interferometer (IASI) observations. It is a simple two-step physical retrieval method, which can be used to relinearize the radiative transfer equation (RTE) by the tangents around the initial estimates of the LST, land surface emissivity (LSE), atmospheric equivalent temperature (Ta), and water vapor content (q) without considering the complex vertical structure of the atmospheric profile. Principal component analysis was utilized to reduce the number of unknown Ta and LSE. The Tikhonov regularization method and discrepancy principle iteration algorithm were employed to stabilize the ill-posed problem and obtain the final maximum likelihood solution of the LST. A new channel selection scheme was proposed for this physical method to obtain an accurate LST estimation. This physical algorithm was tested on both simulated and real data obtained from the IASI. The root-mean-square error (RMSE) of the simulated LST is ~1 K based on an initial LST estimate with an RMSE of 2 K (1.9 K). The sensitivity analysis shows that the LST retrieval accuracy is ~1 K based on an LST with a random error of 3 K, constant initial LSE (0.97), 10% Ta error, and 40% q error. Within the given error range of the initial values of the simulated dataset, the LST retrieval accuracy is insignificantly affected by the initial estimates of the unknown variables. Finally, compared with the Advanced Very High Resolution Radiometer onboard Metop (AVHRR/Metop) LST product, the physical method based on artificial neural network improves the LST retrieval accuracy to 1.5 and 1 K for real daytime and nighttime IASI data obtained in the study area. Based on the new method,