This paper describes a new concept of substrate integrated waveguide (SIW): a slow-wave substrate integrated waveguide (SW-SIW). Compared to a conventional SIW, the proposed topology requires a double-layer substrate with a bottom layer including internal metallized via-holes connected to the bottom conductive plane. The slow-wave effect is obtained by the physical separation of electric and magnetic fields in the structure. Electromagnetic simulations show that this topology of SIW allows decreasing the longitudinal dimension by more than 40% since the phase velocity is significantly smaller than that of a classical SIW. Simultaneously, the lateral dimension of the waveguide is also reduced. By considering a double-layer technology, SW-SIWs exhibiting a cutoff frequency of 9.3 GHz were designed, fabricated, and measured. The transversal dimension and the phase velocity of the proposed SW-SIW are both reduced by 40% as compared to a classical SIW designed for the same cutoff frequency, leading to a significant surface reduction. Moreover, an original kind of taper is proposed to achieve a good return loss when the SW-SIW is fed by a microstrip transmission line.