We present the experimental demonstration of a quasioptical terahertz (THz) detector. It is based on the series connection of three microbolometers. This detector is of high responsivity and broadband response to THz signals. The maximum optical responsivity is at 0.245 THz and the minimum is at 0.367 THz. The thermal time constant of the detector has been demonstrated to be 1.3 μs, which is similar to the ones obtained for single-element microbolometers. These results make arrays of antenna-coupled microbolometers promising for the development of pixels in THz focal-plane arrays.