The Mariana Trench is in the western Pacific Ocean, just east and southeast of the Mariana Islands, between Japan and Papua New Guinea. It lies about 200 km (124 miles) east of the Mariana island chain and southwest of the U.S. territory of Guam.

More specifically, it is:

  • Part of the floor of the North Pacific Ocean, near the boundary where the Pacific Plate is subducting beneath the Philippine Sea Plate.
  • Within the U.S. Exclusive Economic Zone associated with the Northern Mariana Islands and Guam, and much of it is protected as the Marianas Trench Marine National Monument.
  • Roughly crescent-shaped, stretching about 2,550 km (1,580 miles) long and about 69 km (43 miles) wide, with its deepest point, Challenger Deep, located south‑west of Guam.

TL;DR: When people ask “where is the Mariana Trench,” the answer is: in the western Pacific, about 200 km east of the Mariana Islands and near Guam, along a major tectonic plate boundary.