Double-walled carbon nanotubes are between single-walled carbon nanotubes and multiwalled carbon nanotubes. They are comparable to single-walled carbon nanotubes with respect to the light optical density, but their mechanical stability and solubility are higher. Exploiting such advantages, solution-processed transparent electrodes are demonstrated using double-walled carbon nanotubes and their application to perovskite solar cells is also demonstrated. Perovskite solar cells which harvest clean solar power have attracted a lot of attention as a next-generation renewable energy source. However, their eco-friendliness, cost, and flexibility are limited by the use of transparent oxide conductors, which are inflexible, difficult to fabricate, and made up of expensive rare metals. Solution-processed double-walled carbon nanotubes can replace conventional transparent electrodes to resolve such issues. Perovskite solar cells using the double-walled carbon nanotube transparent electrodes produce an operating power conversion efficiency of 17.2% without hysteresis. As the first solution-processed electrode-based perovskite solar cells, this work will pave the pathway to the large-size, low-cost, and eco-friendly solar devices.