We have measured the tune-out wavelength of lithium isotope 7Li at 671 nm. We have used our atom interferometer to measure the phase shift due to the dynamical Stark effect as a function of the laser frequency when a laser beam was focused on a single interferometer arm. The tune-out wavelength is a function of the hyperfine F, m F sublevel, and we have prepared the atoms in the sublevel F = 2, m F = +2 or −2. We find that the tune-out frequency of the F = 2, m F = +2 sublevel is at 3388(8) MHz to the blue of the 2 S1/2, F = 1 → 2 P1/2, F' = 2 transition, the accuracy of this measurement being limited by imperfect optical pumping. This measurement is in excellent agreement with its theoretical value, and, once corrected for different tensorial contributions, our measurement agrees with the more precise measurement of Copenhaver et al. [Phys. Rev. A 100, 063603 (2019)], who also used atom interferometry but different experimental conditions.