Integrated optomechanics finds increasingly broadening applications, requiring tight confinement of photons and phonons within nanometric-scale photonic circuits. However, most existing integrated optomechanical devices use unconventional materials or suspended structures that hinder co-integration with scalable photonic technologies. Here, we show a new optomechanical confinement approach, using subwavelength structuration of silicon to tightly confine near-infrared photons and 600-MHz phonons in nonsuspended silicon waveguides, fully compatible with standard silicon photonics. Indeed, phonons are confined by velocity reduction in silicon and destructive interference of radiation to the cladding, while photons are confined by metamaterial index guiding. We experimentally demonstrate optomechanical microresonators with optical excitation and readout of mechanical modes with a record quality factor of 1120 for silicon-on-insulator devices, measured under ambient conditions and room temperature. The measured optical quality factor is ∼40,000, and the estimated coupling rate is 51 ± 18 kHz. These results are the first step for a new generation of optomechanical devices implemented with scalable silicon photonic technology, having great potential for applications in optical and wireless communications, radar, sensing, metrology, and quantum technologies.