Previous reconstructions of ice-sheet changes in Antarctica’s Weddell Sea sector since the Last Glacial Maximum (LGM) at 19–23 cal. (calibrated) kyr B.P. suffered from large uncertainties and were partly contradictory. As a consequence, the contribution of this sector to the LGM sea-level lowstand and post-LGM sea-level rise was unclear. Furthermore, whether and how precursor water masses for Antarctic Bottom Water (AABW) were formed in the Weddell Sea Embayment under glacial conditions is unknown, as this today requires the existence of the floating Filchner-Ronne Ice Shelf. Here we present new marine geophysical and marine geological data from the outer shelf section of the Filchner paleo–ice stream trough documenting that grounded ice had advanced onto and retreated from the outer shelf prior to 27.5 cal. kyr B.P., i.e., >4500 yr before the LGM. The data reveal the presence of a stacked grounding-zone wedge (GZW) just south of 75°30′S. This GZW was formed during two episodes of grounding-line re-advance onto the outer shelf after 11.8 cal. kyr B.P., with data further inshore implying paleo–ice stream retreat from the GZW location prior to 8.7 cal. kyr B.P. Our findings show that (1) ice-sheet buildup in the Weddell Sea sector made only limited contributions to the LGM sea-level lowstand, (2) ice-ocean interaction below an ice shelf in outer Filchner Trough could have contributed to AABW production at the LGM, and (3) numerical models need to take into account a highly dynamic ice-sheet behavior in regions of the West Antarctic Ice Sheet and East Antarctic Ice Sheet confluence.