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The formation and evolution of the South Pole‐Aitken (SPA) basin is critical to relating large impact basin formation and modification to lunar geophysical evolution. Most prior models of the SPA impact were conducted in 2D, making it difficult to compare model output to the 3D crustal structure and ejecta distribution. In order to better constrain the parameters of the SPA impactor and the expected post impact distribution of crust and ejecta, we conducted numerical simulations of the SPA impact in 3D. We tested a wide range of impact parameters and constrained model results with recent geophysical data. We found the crustal structure of the SPA basin is best fit by an oblique impact (30–45°) of a 350–400 km diameter projectile impacting at 12–16 km/s. The impact excavated material from as deep as 80–120 km, and ejecta was deposited in a butterfly pattern with a forbidden region uprange of the impact. Plain Language Summary: The South Pole‐Aitken (SPA) basin is the largest impact‐generated structure on the Moon. The large size of the basin (approximately 1,500–2,000 km in diameter) implies the impact that generated the basin globally influenced the early evolution of the Moon. In order to constrain the type of impact that formed the basin, we ran computer simulations of large projectiles (300–400 km in diameter) impacting the early lunar surface. We found the structure of the crust surrounding the basin best matches an impact of a 350–400 km diameter projectile (similar in scale to Saturn's moon Mimas) impacting the Moon with a velocity of 12–16 km per second. Our computer models show that the collision that generated the basin would have formed a non‐symmetrical distribution of impact ejecta, and excavated material from the lower crust and upper mantle. Our results constrain the type of impact that formed the SPA basin, and suggest that ejecta from the impact is prevalent at sites of current and future lunar exploration. Key Points: 3D simulations constrain the South Pole‐Aitken (SPA) impactor to 350–400 km diameter impacting at 12–16 km/s and 30–45°The SPA impact excavated lower crust and upper mantle lunar materials, and emplaced ejecta in a butterfly type ejecta patternThe thick crustal annulus present in model results requires further modeling of basin collapse and post‐impact relaxation [ABSTRACT FROM AUTHOR]