Our YBa2Cu3O7−xthin‐film magnetometer consists of a dc superconducting quantum interference device with bi‐epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux‐locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83±3. The low‐frequency rms magnetic field noise scales approximately as 1/f1/2, wherefis the frequency, with a magnitude of 0.6 pT Hz−1/2at 10 Hz and 0.09 pT Hz−1/2at 1 kHz with the magnetometer immersed in liquid N2.