Two-dimensional (2D) metal carbides and nitrides, called MXenes, have attracted great interest for applications such as energy storage. We demonstrate their potential as Schottky-barrier-free metal contacts to 2D semiconductors, providing a solution to the contact-resistance problem in 2D electronics. On the basis of first-principles calculations, we find that the surface chemistry strongly affects Fermi level of MXenes: O termination always increases the work function with respect to that of bare surface, OH always decreases it, whereas F exhibits either trend depending on the specific material. This phenomenon originates from the effect of surface dipoles, which together with the weak Fermi level pinning, enable Schottky-barrier-free hole (or electron) injection into 2D semiconductors through van der Waals junctions with some of the O-terminated (or all the OH-terminated) MXenes. Furthermore, we suggest synthetic routes to control surface terminations based on calculated formation energies. This study enhances understanding of the correlation between surface chemistry and electronic/transport properties of 2D materials, and also gives predictions for improving 2D electronics.