A long-standing issue in the physics of the colossal magnetoresistance is the role of electron-phonon coupling, which manifests itself as Jahn-Teller polarons. The origin and architecture of polarons makes it possible to study their behavior by Raman spectroscopy, which allows to analyze the polaronic behavior in an applied magnetic field. We performed magnetic-field-dependent Raman spectroscopy on thin films of (La _0.6 Pr _0.4 ) _0.7 Ca _0.3 MnO ₃ in a range of H = 0–50 kOe and compared the obtained Raman spectra with the magnetic field behavior of the electrical resistivity. In the vicinity of the Curie temperature, T _C = 197 K, the intensity of the Jahn-Teller stretching mode at 614 cm ⁻¹ and of the bending mode at 443 cm ⁻¹ was found to be suppressed and enhanced, respectively. This observed behavior has a remarkable similarity with the field and temperature dependence of the colossal magnetoresistance in (La _0.6 Pr _0.4 ) _0.7 Ca _0.3 MnO ₃ . Our work provides direct evidence that the reduction of the amount of Jahn-Teller polarons at the phase transition is the main mechanism underlying the colossal magnetoresistance.