Currently, solar absorber plays a key role among all components of the concentrating solar power (CSP) system, which is considered as an important way for solar energy utilization. Due to the outstanding features of surface texture, nano -structured materials have been extensively utilized for solar energy harvesting and conversion, but most of them were synthesized by expensive or complicated techniques. Herein, for the first time, solar selective absorbers with uniquely foamed nanostructure, are grown in situ by facile hydrothermal method on stainless steel, without using any porogen or template. The selective absorber films, which comprised a large number of nanoparticle agglomerates and nanopores, have a vital role on the solar selective absorptance owing to the increased optical path as well as enhanced sunlight trapping via foamed nanostructure. They show an excellent solar thermal performance with the solar absorptance and the thermal emittance of 0.92 and 0.12, respectively. Besides, the solar absorber films also exhibit considerable solar thermal performance and benign thermal stability at high temperature. Due to the relieving internal stresses from foamed nanostructure, there is no obvious cracking within the entire structure even after heat treatment. Consequently, the hydrothermal method used in the present investigation happens to be a novel, pollution-free, low-cost, and suitable for simultaneous mass production for large-size absorber films. Moreover, the pre-treatment of substrate and post-treatment of films are also very convenient and environmental, without any complex procedure. Accordingly, the resultant films with foamed nanostructure will have a good prospect as a new-family of solar selective absorbers, which may have a tremendous potential for industrial production in the future. (C) 2015 Published by Elsevier B.V.