Dark matter remains one of the most profound and unresolved mysteries in modern physics. To unravel its nature, numerous haloscope experiments have been implemented across various mass ranges. However, very few haloscope experiments have been conducted within the millimeter-wave frequency range, which is in the favored mass region for well-motivated dark matter candidates. Here we designed and constructed a millimeter-wave dielectric haloscope featuring a dark matter detector composed of dielectric disks and a mirror. Using this setup, we conducted a search for randomly polarized dark photon dark matter and found no evidence for its existence. Our results established new constraints on the kinetic mixing parameter in the mass range from 387.72 to $391.03\mathrm{\mu }\mathrm{eV}$, improving the existing limits by 2 orders of magnitude. With future enhancements, our system has the potential to explore new parameter space for dark photons as well as axion dark matter within the millimeter-wave frequency range.

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• Particle astrophysics
• Particle dark matter
• Axions
• Dark matter detectors