Common ownership has increased substantially over the past few decades and has become a significant influence on corporate policies. We find that firms owned by the same institution engage in more tax planning. We also observe that the influence of common ownership on corporate tax planning is stronger when the threat of sale is more credible. Further tests show that commonly owned firms exhibit less aggressive income tax reporting and more sustainable corporate tax outcomes. Finally, we revisit inferences from prior literature examining the effect of institutional ownership on tax planning and demonstrate that the magnitudes documented in previous research are strongest when a common owner is present. This latter result underscores the importance of considering the effect of common ownership on corporate tax policies.