•A model was developed for biodynamic responses of the human body to triaxial vibration.•The model included two thighs, pelvis, lumbar spine, middle torso, upper torso, head.•Experimental modal analysis of the human body in sagittal and coronal planes was conducted.•Two of the five modes mainly featured the body vibration in the coronal plane.•The other three mode shapes featured the body vibration in the sagittal plane. While most of the previous studies on biodynamics of the seated human body focused on vertical vibration in the sagittal plane, this study was designed with combined tri-axial translational vibration excitation to investigate the biodynamics in both sagittal and coronal planes. In this study, an analytical model was developed to represent the biodynamic responses of the seated human body to tri-axial translational vibration. The model consisted of thighs, pelvis, lumbar spine (L3 and L4), middle torso (thoracic spine T5-T12 and lumbar spine L1-L2), upper torso (thoracic spine T1-T6), and head (including cervical spine C1-C7). The transmissibilities from the seat to various body locations of subjects sitting with an upright backrest under single-axis and tri-axial vibration were measured. The parameters of the seated human model were determined by fitting the modelled in-line transmissibilities to the chest, L3 and pelvis along the fore-aft, lateral and vertical directions to the measured values. Five vibration modes were identified from the measured transmissibilities below 20 Hz. The first mode at 1.3 Hz featured the lateral sway of the upper body in the coronal plane. The second mode at 2.5 Hz characterised the lateral motion of the whole body accompanied by the lateral bending motion of the torso. The third mode at 3.6 Hz contained the fore-aft and pitch movements of the upper body with pitch of the pelvis. The fourth mode at 6.4 Hz included the vertical motion of the entire body with pitch of the pelvis, and the fifth mode at 10.3 Hz involved the vertical motion of the thighs with pitch and vertical motion of the pelvis. The model was verified with the measured vibration modes. This study provided a method for the identification of model parameters using the body transmissibilities and modal properties.