We design, fabricate, and characterize a novel type of wavelength-flattened directional coupler (WFDC) working in the mid-infrared (MIR) based on the physics of rib waveguide dispersion. In the silicon-on-insulator rib waveguide WFDC devices with length <20 μm, a 6-fold enhancement and a 4-fold enhancement in the operation bandwidth compared with the conventional directional coupler are achieved for 50:50 (±5) and 100:0 (−2) power splitting ratio, respectively, with an average lowexcess loss of −0.52 ± 0.18 dB/device. To the best of our knowledge, our device is the first WFDC working in the MIR and the first WFDC that possesses low excess loss, CMOS compatibility and compactness simultaneously, while the novel mechanism could be adopted easily to realize arbitrary power splitting ratio. Our work could serve as a promising component for light routing and power splitting in broadband MIR applications, such as MIR spectrometer sensing systems. In addition, the proposed novel mechanism could be adopted for near-infrared as well to achieve better WFDC performance.