Longleaf pine (Pinus palustris Mill.) forest is a well-known fire-dependent ecosystem. The historical dominance of longleaf pine in the southeast United States has been attributed to its adaptation known as the grass stage, which allows longleaf pine seedlings to survive under a frequent surface fire regime. However, factors affecting post-fire survival of grass stage seedlings are not well understood. In this study, we measured live and dead longleaf pine grass stage seedlings to quantify the role of seedling size, root collar position, and sprouting in seedling survival following a wildfire in the sandhills of South Carolina. We found that fire resulted in almost 50% mortality for longleaf pine grass stage seedlings. Fire survival rate increased with seedling size, but a size threshold for fire tolerance was not supported. Fire survival depended on the position of root collar relative to the mineral soil. Seedlings with protected root collars (i.e., buried in or at the level of mineral soil) experienced <21%, while seedlings with exposed root collars (i.e., elevated above mineral soil) suffered >90% post-fire mortality. Ability to resprout contributed to 45.6% of the total fire survival, with the small seedlings (root collar diameter (RCD) < 7.6 mm) almost exclusively depending on resprouting. Our findings had significant implications for fire management in longleaf pine ecosystems, and the current frequency of prescribed fire in sandhills might need to be lengthened to facilitate longleaf pine natural regeneration.