Abstract: Unbonded concrete overlay (UBCO) is the most used pavement rehabilitation technique across United States with an overall usage of 47% amongst all the rehabilitation methods. It is aimed to improve the performance of any deteriorated/cracked jointed plain concrete pavement (JPCP). Unbonded JPCP overlay involves placement of a separation layer of hot mix asphalt (HMA), which acts as a stress relief layer between the existing concrete pavement and the overlay. There are numerous factors that affect the design and performance of UBCOs and out of these, existing pavement condition or the severity of damage of existing pavement is a prime factor. The severity of damage is described by the distressed elastic modulus thus, accurate determination of the distressed elastic modulus of the existing concrete pavement is essential for predicting the accurate performance of the unbonded overlay. This study focuses on analyzing the impact of distressed modulus by conducting simulations in the AASHTOWare pavement ME design software version 2.3 and evaluating the predicted performance of JPCP overlay for two different climatic regions. The results indicated that the distressed modulus of existing concrete pavement affects the performance of the overlay with regards to transverse cracking, joint faulting and pavement roughness. Transverse cracking is the most affected performance parameter with a change of 0.27%–2.31% with a unit change in distressed modulus. The impact of climatic conditions on the performance of unbonded overlay was also observed. The adverse effects of distressed modulus can be minimized by reducing the joint spacing or increasing the overlay slab thickness.