A combined experimental and computational approach was adopted to determine a constitutive relationship for mesenchymal gap tissue generated during distraction osteogenesis. Harvested distraction zones were tested to failure in uniaxial tension and 3-D FE models were constructed to simulate mechanical testing. A specific form for the strain energy function was adopted to obtain strain-stiffening behavior. Theoretical uniaxial tension predictions were consistently lower than those from 3-D FEA. By refining the values of the three material parameters, a force-displacement curve in excellent agreement with the experimental results was obtained. This formulation appears capable of capturing the essential elements of the behavior of distraction gap tissue. These results can be implemented to predict strain distributions within this tissue and improve our understanding of how the mechanical environment affects bone regeneration and precursor tissue differentiation during distraction.