The soft ferroelectric phonon in SrTiO3observed with optical spectroscopy has an extraordinarily strongspectral weight which is much stronger than expected in the limit of a perfectly ionic compound. This “chargedphonon” effect in SrTiO3is caused by the close-to-covalent character of the Ti-O ionic bond and implies a strongcoupling between the soft ferroelectric phonon and the interband transitions across the 3-eV gap of SrTiO3.Wedemonstrate that this coupling leads, in addition to the charged phonon effect, to a pairing interaction involvingthe exchange of two transverse optical phonons. This process owes its relevance to the strong electron-phononcoupling and to the fact that the interaction mediated by a single transverse optical phonon vanishes at lowelectron density. We use the experimental soft phonon spectral weight to calculate the strength of the biphononmediated pairing interaction in the electron-doped material and show that it is of the correct magnitude whencompared to the experimental value of the superconducting critical temperature. Biphonon exchange is thereforean important pairing mechanism at low doping, and may be the key to understanding the occurrence ofsuperconductivity in doped SrTiO3and other low electron density materials.