We report the control of tunnel ionization in lithium niobate (LiNbO3) using phase-controlled two-color laser fields. Through a macroscopic observable of high contrast, we disclose the crucial contribution of the microscopic spontaneous polarization of the ferroelectric material to the ionization rate: As the relative two-color phase is varied, the ablated area of LiNbO3 is modulated by 35% when the laser and crystal polarization directions are parallel. Rotating the sample by 180â around the laser propagation axis leads to an out-of-phase modulation. We use a two-band model to highlight the key contribution of the material's spontaneous polarization for the symmetry breaking of the ionization rate. Our results open new perspectives for the direct control of ionization dynamics in solids by tailoring the electric field of femtosecond laser pulses.