We have developed a new methodology to study the effect of mechanical stress on spin waves in thin films deposited onto compliant substrates. It is based on micro-tensile tests combined with Brillouin light-scattering spectroscopy, which allows in situ probing of the magnetization dynamics of the studied film upon deformation. This paper shows from both theoretical and experimental approaches that the magneto-elastic coupling in the saturation regime leads to a simple linear relationship between the spin waves' frequency and the stress applied to the magnetic film. The linear part of the experimental data can be reproduced theoretically, assuming a complete strain transfer through the metallic film–compliant substrate interface.