A new type of sub‐keV x‐ray (0.1–1.2 keV) spectrometer diode array time‐resolved transmission grating spectrometer (DATTS) with a fast time response (≤270 ps) has been developed to measure transient soft x‐ray spectra from laser‐produced plasmas. DATTS, the combination of the transmission grating (TG) and fast biplanar diodes, offers many advantages over the conventional x‐ray diagnostics. Among the advantages are flexibility of the energy windows, fast time response, large dynamic range, and the capability of measuring absolute x‐ray energy. All of these measurements have not been possible with a single diagnostic. The performance of the DATTS was compared with the data taken with the x‐ray diode system, showing excellent agreement in the sub‐keV x‐ray energy range.