A monolithic ridged half-mode waveguide bandpass filter with a periodic defected ground array has been successfully fabricated. Utilizing the high-pass characteristic of the waveguide and the rejection band generated by the periodic defected ground structure, an effective passband was created. The measured results show a 3 dB passband from 39.4 to 45.4 GHz, centered at 42.4 GHz with a 3-dB fractional bandwidth of 14.1%. The lowest insertion loss of 2.4 dB locates at 44.2 GHz. The suppression in upper rejection band reaches 40 dB at 58 GHz. Compared to conventional rectangular waveguide filters, the width of the proposed filter is reduced by 64%, which benefits to the integration and miniaturization. With the development of the next generation wireless communication (5G) towards millimeter-wave band, the miniaturized millimeter-wave filter has promising potential for 5G communication.