Background. Hand palpation is a conventional way to assess and document soft tissue fibrosis. But it is semi-quantitative and subjective, so there is a need to develop quantitative and objective methods for this purpose. ; Methods. 105 patients with different degrees of radiation-induced fibrosis of soft tissue of the neck were assessed using an ultrasound indentation method. The force response was reconstructed from the indentation history using a quasi-linear viscoelastic model with four material parameters. The parameters which best curve-fitted the force response with respect to the experimentally measured one, were selected as the viscoelastic properties of the tested soft tissue. These parameters were compared among patient subgroups with different degrees of fibrosis as scored by hand palpation, and also compared with those of a control group of healthy, non-irradiated subjects. Their relation to the rotation range of the neck and the effective Young’s modulus, were also assessed. ; Findings. Soft tissue with a more severe degree of fibrosis was associated with a larger initial stiffness and a more rapid increase in stiffness under loading. Viscoelasticity parameters could discriminate soft tissue with different degrees of clinical fibrosis and had significant correlation with clinical parameters of fibrosis. ; Interpretation. Change of viscoelastic properties is reflection of pathological modifications of components in fibrotic soft tissues. Measurement of viscoelasticity parameters of soft tissue provides a quantitative and objective approach for the researcher and clinician to quantify soft tissue fibrosis. ; Author name used in this publication: Sing-Fai Leung