Terahertz (THz) detectors have drawn much attention and have been widely applied in imaging, spectroscopy, and sensing fields. An uncooled monolithic resonant THz thermal detector implemented in a standard 55-nm CMOS process is presented. The integration of a frequency selective metamaterial (MM) absorber coupled with an optimized proportional-to-absolute temperature (PTAT) sensor leads to an approach toward uncooled, compact, low-cost, easy-integration, and mass-production THz detectors. The theoretical analysis, design considerations, and characteristic measurements are demonstrated in detail. The proposed thermal detector is designed to resonate at 2.58 THz for the accessible THz source and the natural atmospheric window. The MM absorber achieves near-perfect absorptivity of 98.6%, and the optimized PTAT sensor obtains a high-temperature sensitivity of 10.11  mV  /    °  C. The calculated responsivity is 49.81  V  /  W at 2.58 THz with a calculated noise-equivalent power (NEP) of 0.78  μW  /  Hz^0.5 at a modulation frequency of 3 Hz. Relatively better experimental results are obtained at 2.58 THz with a maximum responsivity of 48.3  V  /  W and a minimum NEP of 1.06  μW  /  Hz^0.5.