Very sharp 90°micro-bends for adiabatic optical wave propagation in multimode waveguides are demonstrated by an inverse design method, and the devices are fabricated on a silicon-on-insulator platform. The compact bending structures are based on digital waveguide metastructures with footprints as small as 2.6 μm×2.6 μm. For waveguides with widths of 2 μm and bending radii of only 1 μm, the TE00 mode is able to turn around the sharp corner with 1 dB loss and >20 dB suppression ratio to higher-order modes. The transmission spectra of the devices are measured with ∼1 dB loss over a 40 nm bandwidth which are consistent with the numerical simulations.