The properties of either carbon nanotubes (CNTs) or nanoribbons (CNRs) in their pristine form are nevertheless insufficient to satisfy the increasing demands of various applications. Strategies that can endow these carbon-based nanostructures with guided functionalization are in urgent need. Herein, we theoretically demonstrate that carbon nanoribbons can be welded by a variety of metal-atoms, such as alkali metals, III-IV group metals, and transition-metals, to form functionalized metal-welded carbon nanotubes (MW-CNTs), which represent a new family of carbon-based nanostructures. It is significant that the metal-welded CNTs exhibit noticeably lower formation energies than their nanoribbon counterparts, indicating that this new family of carbon nanostructures can be synthesized experimentally. The introduction of the hetero-metal-atoms endows MW-CNTs with fascinating tailored properties. For example, in the 3d magnetic transition-metal-welded (Cr, Mn and Fe) nanotubes, Cr-welded CNTs show half-metallic properties, giving them potential applications in spintronic or magnetic devices; while Fe-welded CNTs display superior catalytic activity towards the dissociation of water molecules. The salient electronic and catalytic properties of this novel family of metal-welded carbon nanotubes pave the way to design high-performance devices for energy harvesting, conversion and storage. More importantly, the idea of heteroatom welding of foldable two-dimensional systems may develop into a versatile design strategy that can be extended to BN, MoS2, TiO2, and other two-dimensional (2D) nanosheets.