We describe the design, schematics, and performances of a low‐noise preamplifier for pulsed nuclear‐magnetic‐resonance (NMR) experiments in the range 0.1–1 MHz with a noiseless (cooled below liquid‐helium temperature) pickup coil. This preamplifier operates at room temperature and exhibits an order‐of‐magnitude improvement in noise power per unit bandwidth over previously used preamplifiers. All significant contributions to the noise are carefully identified and a detailed analysis of the noise performance of a cascode differential pair of JFETs is given. Application to very‐low‐temperature thermometry by NMR in pure platinum powder is discussed as a practical example.