The coupling of spin waves with charge- and spin-density waves is shown to be induced by a spin-dependent interaction in a quantum well, which is spin polarized by a dc magnetic field at an angle ␪ to the symmetry axis. The mixing of the plasmonic and magnonic modes, which occurs for both intra- and intersubband transitions, depends on the coupling constant of the spin-spin interaction, the tilt angle ␪, and the initial spin polarization ␨. ͓S0163-1829͑99͒08247-8͔