Integrated acousto-optic (AO) devices utilize the strong overlap of acoustic and optical fields in a waveguide to facilitate efficient photon–phonon (Brillouin) interactions. For example, acoustic waves offer a lossless modulation mechanism for light. "Brillouin active" photonic platforms are currently being developed that may see optical, acoustic, and AO waveguide circuits on the same chip, where guided light and sound come together in active interaction regions. A key missing component for such a platform is a device that can multiplex modes across these two physical domains. We propose and describe a new class of optical and acoustic components, the "acoustic–optical mode multiplexer" (AOMM), a device that takes respective optical and acoustic waveguides as input ports and couples their excited guided modes into a single, joint output waveguide. We show an example suspended silicon–silicon dioxide design that combines two optical modes and a spatially separate acoustic mode into a single, co-guided output port with low insertion loss down to 0.3 dB for both optical and acoustic modes, and reflection below − 20 d B and − 11 d B , respectively. The AOMM may enable new, efficient integrated AO devices, such as isolators and circulators, where the acoustic wave generation and opto-acoustic interaction are separated.