Using radar satellite data from the Italian COSMO‐SkyMed (CSK) constellation and the German TanDEM‐X formation, we present comprehensive measurements of the biweekly grounding line dynamics of Pine Island Glacier, West Antarctica, from August to December 2015. The 1 day repeat cycle of CSK reveals tidally induced, grounding line migration on the scale of kilometers and extensive seawater intrusion within the grounding zone, which significantly exceeds that predicted for a stiff bed but are consistent with that calculated for a deformable bed. The deformable bed also explains the continuous draining/filling of subglacial lakes proximal to the grounding line. After correction for oceanic tides, we estimate a retreat rate for 2011–2015 of 0.3 km/yr at the glacier center and 0.5 km/yr on the sides, which is 3 times slower than for 1994–2011 (1.2 km/yr at the center). We attribute the decrease in retreat rate to colder ocean conditions in 2012–2013 relative to 2000–2011. Plain Language Summary Using short‐term repeat satellite data from the Italian CSK constellation, we document significant tidal migration of the junction between ice and the ocean on the Pine Island Glacier, West Antarctica, which is the largest contributor to sea level rise from Antarctica. The results imply that the junction is not fixed but corresponds to a zone several kilometers wide where ocean water melts ice from below. At present, numerical models of ice flow do not account for a flush zone. The existence of a flush zone should make the glacier more prone to retreat in response to warmer ocean waters. Key Points We observe significant sort‐term grounding line dynamics of a major glacier in west Antarctica Faster retreat along shallow bed slopes helps validate bed topography models Short‐term dynamics implies the presence of a soft bed, a high sensitivity to tides, and flushing of seawater within the grounding zone