A broadband and scalable mode multiplexer (MUX) using an adiabatic coupler (AC) assisted with subwavelength grating (SWG) is proposed and analyzed in detail. The adiabatic coupler consists of a strip bus waveguide and a tapered waveguide whose width decreases along the propagation direction so that the fundamental TE mode cannot be supported well and will be coupled into the high-order mode in the bus waveguide via mode evolution due to the phase matching. On the contrary, the fundamental TE mode injected into the bus waveguide will keep propagating in the same waveguide with negligible coupling. The SWG is incorporated into the gap between the bus and tapered waveguides for effectively enhancing the mode coupling, leading to a reduction in length of the device. By carefully choosing the structural parameters, the proposed TE₀  /  TE₁ mode MUX shows a low insertion loss (IL) of 0.07 dB (0.01 dB) and crosstalk (CT) of −24.01  dB (−21.25  dB) for the coupled TE₁ mode (the through TE₀ mode) at wavelength 1550 nm while the coupling length is compact as ∼8.8  μm. The device achieves a broad bandwidth of 165 nm for CT  <    −  19  dB and IL  <  0.11  dB over a wavelength range of 1460 to 1625 nm, covering the whole S, C, and L bands. Moreover, the device also shows a good fabrication tolerance to the deviations of the waveguide width, the total height of the device and the grating duty cycle. In addition, the TE₂ and TE₃ multiplexing are also presented, showing the scalability of the proposed scheme.