Excitons are strongly correlated pairs of electrons and holes and dominate the absorption spectrum of semiconductors near the bandgap energy. The intraexcitonic transition energy lies in the terahertz (THz) frequency range with large dipole moments, making exictons useful systems not only for studying the fundamentals of nonlinear optics but also for exploiting THz excitonic interactions in optical communication devices. However, so far, the study of the effect of excitonic dressed states on the optical field has been limited because of the usage of a single optical frequency as a probe. The strong electric field with a stable phase that is associated with optical pulses may make it possible to investigate the role of the dressed state. Here, we probe the transient absorption changes of a near-infrared (NIR) pulse in a GaAs quantum well in the presence of a multi-cycle THz wave. By changing the delay between the NIR probe and the THz wave, the absorption strengths can be modulated on a sub-cycle THz timescale, and the frequency analysis shows the formation of THz-induced dressed states of excitons.