Semiconductor nanowire (NW) lasers are nanoscale coherent light sources that exhibit a small footprint, low-threshold lasing characteristics, and properties suitable for monolithic integration onto Si photonic circuits. An important milestone on the way towards novel on-chip photonic functionalities is the integration of individual, deterministically addressable NW lasers on Si waveguides with efficient coupling and mode propagation in the underlying photonic circuit. Here, we demonstrate the monolithic integration of single GaAs-based NW lasers directly onto lithographically defined Si ridge waveguides (WG). Subject to optical excitation, the observed lasing behavior shows clear “s-shape”-characteristics, linewidth narrowing and threshold values down to 19.8±1 µJ/cm², which is the lowest value reported to date for this kind of integrated lasing structure. The lasing mode of individual NW lasers is shown to couple efficiently into propagating modes of the underlying orthogonal Si WG, preserving the spectral characteristics during mode pro-pagation in the WG, and in good agreement with Finite-Difference Time-Domain (FDTD) simulations. Using a WG structure with a series of mask openings along the central mode propagation axis, we further illustrate the out-coupling properties of both spontaneous and stimulated emission and demonstrate propagation of the lasing mode over distances > 60 µm, despite absorption in the silicon dominating the propagation losses. By replacing the bulk GaAs as the active gain medium by InxGa1-xAs quantum wells, the emission wavelength can be shifted towards telecommunication bands in order to reduce the absorption losses in silicon. These results pave the way for future on-chip monolithic integration of III-V NW lasers onto silicon based optoelectronic circuits.