We employ time-resolved resonant x-ray diffraction to study the melting of charge order and the associated insulator-to-metal transition in the doped manganite Pr 0.5 Ca 0.5 MnO 3 after resonant excitation of a high-frequency infrared-active lattice mode. We find that the charge order reduces promptly and highly nonlinearly as function of excitation fluence. Density-functional theory calculations suggest that direct anharmonic coupling between the excited lattice mode and the electronic structure drives these dynamics, highlighting a new avenue of nonlinear phonon control. DOI: 10.1103/PhysRevLett.118.247601 Some of the most fascinating phenomena in condensedmatter physics arise from electron-phonon interactions. A striking example is the BCS theory for superconductivity, where phonons mediate an attractive interaction between two electrons [1]. In metals and semiconductors, transport properties are also shaped by linear electron-phonon (e-ph) coupling, in particular through the creation of polarons [2]. Furthermore, e-ph coupling plays an important role in the physics of perovskite oxides such as the mixed-valence manganites, where a variety of electronic and magnetic phases is stabilized via the Jahn-Teller effect [3]. The precarious equilibrium between these possible ground states is easily perturbed by external stimuli such as static electric or magnetic fields, temperature, pressure, or even by light, opening new ways of manipulating matter on ever faster time scales with short laser pulses [4][5][6][7][8][9][10].The recent progress in the generation of high-energy ultrashort pulses in the midinfrared (mid-IR) range permits us to resonantly excite vibrational modes of the lattice to large amplitudes, exceeding several percent of the interatomic distances-a value at which nonlinear coupling of phonons to other degrees of freedom becomes important [11][12][13]. This approach enabled mode-selective material control on subpicosecond time scales, as exemplified in vibrationally induced superconductivity [8,12] and insulator-metal transitions [11], as well as lattice-driven suppression of magnetic and orbital order [14]. A proposed mechanism, referred to as "nonlinear phononics" and based on nonlinear phonon-phonon coupling, suggests that these phenomena result from the rectification of the excited mode and a net displacement of the crystal lattice along the coordinates of anharmonically coupled vibrational modes, which control the electronic properties [8,[14][15][16]. A possible direct coupling between the excited mode and the electronic system has only seldomly been considered [17], although, for example, the vibrational excitation of a molecular solid was shown to coherently perturb electronic interactions [18].In this Letter, we explore the nonlinear electron-phonon coupling in a manganite by investigating the lattice driven ultrafast insulator-metal transition [11]. We use resonant xray diffraction at the Mn K edge to study the dynamics of the electronic and structural order in a Pr 0.5 Ca 0.5 MnO 3 (PCM...