Iron hydrides attract significant interest as candidates for the main constituents of the Earth's core in geophysics and planetary science. However, their basic physical properties are still not well known. Here, we combined high-pressure transport, synchrotron radiation Mössbauer, and Fe Kβ x-ray emission spectroscopy measurements on ′-FeH to map out a detailed magnetic phase diagram of this hydride phase of iron. In contrast to our original expectations, we found two magnetic phase transitions at high pressure due to two inequivalent iron sites existing in the ′-FeH structure. Our results account for the previous large pressure difference on the loss of ferromagnetism between experiment and theoretical calculations. The discovery of an unexpected complex magnetic phase diagram in ′-FeH has implications for a better understanding of the magnetic and physical properties of the iron-hydrogen compounds, important for the conditions of planetary interiors.