Multifrequency synthetic aperture radar (SAR) data have been applied to discriminate subtle differences in the vegetation and to better characterize its structural properties, since each SAR frequency will interact with the different sections of the vegetation canopy. In this study, our main objective was to evaluate the use of multifrequency Sentinel-1 and ALOS-2/PALSAR-2 data for stem volume estimations in Eucalyptus sp. and Pinus sp. plantations using three different machine learning algorithms: random forest (RF), support vector regression (SVR), and extreme gradient boosting (XGB). Different experiments were carried out using combinations of predictor variables derived from both SAR sensors: backscattering, polarimetric decompositions, and interferometry data, and field data considering specific models for Eucalyptus sp. and Pinus sp. and a generic model comprising all forest plantations data. The machine learning models using predictor variables derived from SAR data achieved moderately high accuracy to predict stem volume, mainly when SAR data were used in combination with stand age (Experiment iv). In the best prediction scenario (Experiment iv), the RF, SVR, and XGB models were able to explain 81.7%, 68.5%, and 81.8% [coefficient of variation (R²) values] of stem volume variability considering the generic models, respectively. Our results pointed out that the RF algorithm showed the best performance in predicting stem volume with significant good results and easier implementation in comparison with the other two algorithms (SVR and XGB).