We provide the first large-sample evidence of banks playing an important role in facilitating tax planning of their client firms. We posit that banks act as tax planning intermediaries, developing and implementing tax strategies for corporate clients and by facilitating such strategies’ spread across firms. Capturing bank-client relationships using lending contracts, we find that a client’s own tax planning is strongly associated with the average tax planning of its bank’s other clients. Further tests using new banking relationships show that clients experience meaningful reductions in their taxes when they begin a relationship with a bank whose existing clients engage in above-median tax planning. The effect is stronger when the new bank has above-median investment banking activities. Moreover, the effects are concentrated in clients with foreign income and those with greater credit risk and in relationships characterized by longer periods and larger lending facilities. Our results suggest that while banks are financial intermediaries, they also act as tax planning intermediaries, facilitating tax planning across their relationship networks.