A new, fast, and very sensitive semiconductor bolometer has been designed and developed for the measurement of radiative energy losses in fusion devices in which the hard radiation level is not too high. Special care was taken in the design of the bolometer to simplify both the construction technology and the installation in fusion devices. The major novelty of this bolometer is the use of a low thermal conductivity rigid substrate, instead of the typical thin foil stretched on a rigid frame. In spite of large detector‐substrate contact area, the cooling time constant reaches values as high as 150 ms. The rigid substrate allows the detector to be very robust and reliable; moreover, the monolithic integration of array structures is straightforward. The detector has been fabricated with a thin‐film multilayer technology achieving very low thermal capacitance (<2 mJ/K) and very low response time (<5 &mgr;s). High responsivity (20 V/W) is obtained using GeOxthin film with a high thermal coefficient of the electrical resistance (−4.5%/K) as sensing thermistor.