The design of cross-laminated timber (CLT) shear walls demands careful consideration. In order to reduce cost and speed up construction time, large panel sizes are often preferred, yet sometimes openings are required for architectural purposes. However, the Canadian Standard for Engineering Design in Wood (CSA O86-19) prohibits openings in CLT shear walls due to a lack of research quantifying the reduction of stiffness as a function of opening size. To address this research gap, the impact of opening size on the stiffness of CLT panels was investigated. A total of 43 tests were conducted on panels with two different aspect ratios, two different thicknesses, and various opening sizes. The results indicate that the panel stiffness decreases non-linearly with an increase in opening size. However, even opening sizes large in relation to the panel width have only a minimal impact on the panel stiffness. These findings provide valuable insights for future design provisions for openings in CLT shear walls.