We propose and demonstrate a subwavelength hole defect assisted microring resonator (SHDAMR) structure. With the manipulated modal coupling between two degenerate counterpropagating modes induced by a subwavelength hole defect embedded in the microring waveguide, the SHDAMR structure shows a rectangular resonance lineshape instead of the Lorentzian resonance lineshape of a conventional microring. As a proof of concept, the SHDAMR structure is fabricated on the Si3N4 waveguide platform, for achieving a rectangular filter with a 3-dB bandwidth of 2.03 GHz and an improved shape factor. The demonstrated SHDAMR structure shows the advantages of compact footprint, simplified tunability, and large tolerance of fabrication errors, showing great potential for various applications.