We propose a simple multiaxial linear viscoelastic constitutive model to describe the behavior of thermo-rheologically complex polymers in both the time and frequency domains. The numerical implementation is discussed along with the parameter identification procedure. In particular, we propose a modification of the parameter identification procedure proposed by Jalocha et al. (2015) introducing a regularization step, enabling robust handling of large experimental datasets. We perform an extensive experimental investigation, including calorimetry analysis, as well as tensile, shear, and torsion frequency sweep, creep, and stress-relaxation experiments, and results are benchmarked against existing literature. The frequency-temperature superposition principle is applied to the experimental data using horizontal and vertical shift functions, explicitly integrated into the constitutive model framework. The calibration of the constitutive model is performed in the frequency domain, while the validation is performed in the time domain, demonstrating the accuracy and reliability of the model in multiple experimental domains. Finally, model predictions are reported in the frequency domain for different temperatures. • We developed and validated a constitutive model for viscoelastic behavior of complex polymers. • Calorimetry and mechanical tests on polypropylene were performed and compared with literature data. • A multiaxial linear viscoelastic model for complex polymers is presented in time and frequency domains. • Regularization was added to the identification procedure to handle large experimental datasets. • Frequency-temperature superposition was applied using horizontal and vertical shift functions in the model. • The model was calibrated in the frequency domain and validated with stress-relaxation experiments.