Systems, devices, and methods for transvascular ablation of target tissue. The devices and methods may, in some examples, be used for splanchnic nerve ablation to increase splanchnic venous blood capacitance to treat at least one of heart failure and hypertension. For example, the devices disclosed herein may be advanced endovascularly to a target vessel in the region of a thoracic splanchnic nerve (TSN), such as a greater splanchnic nerve (GSN) or a TSN nerve root. Also disclosed are methods of treating heart failure, such as HFpEF, by endovascularly ablating a thoracic splanchnic nerve to increase venous capacitance and reduce pulmonary blood pressure.