A new acoustic metamaterial plate (AMP) is proposed herein. The plate incorporates Helmholtz resonators that are periodically embedded at intervals shorter than acoustic wavelengths. This metamaterial plate exhibits extraordinary sound transmission loss (STL) at the resonance frequency of the Helmholtz resonators compared to a conventional flat plate. The STL of the AMP can be theoretically analyzed using the effective mass density and flexural rigidity. At the resonant frequency, the dynamic density of the AMP becomes much larger than that of a conventional solid flat plate with the same mass. When the Helmholtz resonant frequency is tuned to the coincidence frequency of the AMP, the dip in transmission loss owing to the coincidence effect is not observed. The frequency band, wherein high STL occurs, is narrow; however, the frequency band can be widened by embedding multiple resonators with slightly different resonant frequencies. Numerical experiments are also performed to demonstrate the acoustic performance of the proposed system. In the simulation, Helmholtz resonators with the 2.1-kHz resonant frequency are embedded at 20-mm intervals inside a 6-mm-thick flat glass plate. Analytical solutions of this system agree well with numerical solutions for various incidence angles of incoming plane waves. In this configuration, we find that the degradation of STL caused by the coincidence effect is nearly eliminated for waves that are incident at random angles.