<p>Dynamical control of the nonlinear optical properties of solids – with light itself – will be essential for future ultrafast photonic technologies. Previously, methods to modulate nonlinear processes including second-harmonic generation (SHG) have relied primarily on non-resonant light-matter interaction or photo-generation of hot electrons in nanoscale materials. However, these approaches are typically constrained by limited interaction lengths and the initial frequency conversion is relatively weak under equilibrium conditions. Here, a ~ 30% modulation of efficient phase-matched SHG in bulk beta-barium borate (β-BaB₂O₄) is achieved through transient lattice deformation by intense terahertz (THz) pulses that are tuned to resonance with an infrared-active phonon mode. The effect originates from modification of the index of refraction ellipsoid and the corresponding nonlinear phase-matching conditions, rather than from direct modulation of the nonlinear susceptibility through THz-mediated <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\chi }^{(3)}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mi>χ</mi> </mrow> <mrow> <mrow> <mo>(</mo> <mrow> <mn>3</mn> </mrow> <mo>)</mo> </mrow> </mrow> </msup> </math></EquationSource> </InlineEquation> processes. This mechanism, of resonant selective lattice excitation, points toward novel THz-control schemes to tune the nonlinear optical response in materials.</p>