On the 18th, she loaded cargo and troops of the 4th Marine Division and, on the 29th, got underway with TG 52.15 bound via Eniwetok for the assault on the Marianas. Late on the 14th, she approached Saipan. In the dark predawn on the 15th, exploding star shells lighted the sky on "Thuban's" port beam. Ships of the screen reported the sinking of a submarine as "Thuban" steamed around the northern tip of the island and headed south, sighting Saipan for the first time at 0445. Before sunrise, she lay to in the transport area and began lowering her boats. As the first assault waves hit the beach in the faint light of early dawn, "Thuban's" crew enjoyed a grandstand view of "D-day" on Saipan, especially of the shoreline south of Charankanoa where her own landing craft were operating.