The recent discovery of high-temperature superconductivity in pressurized nickelates has renewed interest in the broken-symmetry states of their ambient-pressure parent phases, where a density wave (DW) order emerges and competes with superconductivity, but its microscopic origin remains unresolved. Using ultrafast optical spectroscopy, we track quasiparticle relaxation dynamics across the DW transition at $T_{\mathrm{DW}}\approx 136\text{K}$ in trilayer nickelate ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ single crystals, revealing the opening of an energy gap of $\sim 52$ meV. Multiple coherent phonons, including $A_g$ modes near 3.88, 5.28, and 2.09 THz, display pronounced mode-selective anomalies across the transition, indicating that the DW is strongly coupled to lattice degrees of freedom and suggesting an important role of electron-phonon coupling. At higher excitation densities, the DW is nonthermally suppressed, producing a temperature-fluence phase diagram that parallels pressure-tuned behavior. These results establish the DW in ${\mathrm{La}}_4{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ as a lattice-entangled instability involving multiple phonon modes, and highlight ultrafast optical excitation as a nonequilibrium tuning parameter for suppressing density wave order in nickelates.

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• Charge density waves
• Electronic structure
• Spin density waves
• Superconductivity
• Optical spectroscopy
• Reflectivity
• Ultrafast femtosecond pump probe