We report a simple method for the photothermal welding of nonwoven mats of electrospun nanofibers by introducing a near-infrared dye such as indocyanine green (ICG) into the nanofibers. By leveraging the strong photothermal effect of the dye, the nanofibers can be readily welded at their cross points or even over-welded (i.e., melted and/or fused together) to transform the porous mat of nanofibers into a solid film upon exposure to a near-infrared laser. While welding at the cross points greatly improves the mechanical strength of a nonwoven mat of nanofibers, melting and fusion of the nanofibers can be employed to fabricate a novel class of photothermal papers for laser writing or printing without involving any chemicals or toner particles. Through the use of a photomask, we can also integrate photothermal welding with the gas foaming technique to pattern and then expand nonwoven mats of nanofibers into three-dimensional scaffolds with well-defined structures. This method can be applied to different combinations of polymers and dyes, as long as they can be co-dissolved in a suitable solvent for electrospinning.