Ōsumi-class tank landing ship

The Ōsumi class LST (おおすみ型輸送艦), is a Japanese amphibious transport dock. The class is also known as the Oosumi class. While the JMSDF describes the Ōsumi class as tank landing ships, they lack the bow doors and beaching capability traditionally associated with LSTs. Functionally, their well deck makes the Ōsumi class more like a dock landing ship (LSD) or amphibious transport dock (LPD).

Globalsecurity.org noted in its report on the Ōsumi class that "the program originated in a proposal for a small carrier for defensive and mine countermeasures (MCM) purposes, but this was deemed politically unacceptable, and the project was reworked as an amphibious ship" (actually a “Maritime Operational Transport", see below). Topside the open air vehicle parking deck gives the appearance of a small aircraft carrier though the helicopter flight deck only comprises the rear area of the ship and the small deck elevators are for vehicles rather than helicopters with a parking area below with access to the well deck rather than an aircraft hangar.

Later the JMSDF returned to the idea with helicopter carriers with the larger Hyūga class.

The MSDF developed the Maritime Operational Transport concept as an alternative to what was then (mid-1990s) the politically denied development of an amphibious doctrine. This concept is intended to "deliver JGSDF reinforcement units to an area where an enemy landing is possible or probable, or where an enemy has already landed but that is still under Japanese control. The point is that the landings would be on Japanese territory, not foreign soil. So, in theory, this concept does not involve amphibious assault. The tempo of helicopter transport and the types of helicopters required would be very different from those in an assault amphibious landing."

Osumi gives the JMSDF a modest lift capability, especially in defense of the outer islands. The "Ohsumi" type of vessel enables the efficient transport of JDS ground troops to strategic locations, and the rescue of civilians in case of large scale natural disaster. The massive hull features armored and opening deck. The rear of hull houses an armored landing deck for two large CH-47 helicopters. The well-dock in the rear of the ship houses the two Landing Craft Air Cushion (LCAC) hovercraft. Forward, there is a 100m deck below the main deck, but the single elevator accessing these spaces is too small for anything but a small helicopter. A VSTOL aircraft or a larger helicopter will simply not fit on the elevator. These spaces below the forward main deck are used primarily for vehicle storage so those fighting and/or armored vehicles can access the well deck. Perhaps one or two small helicopters could be stored and serviced there, such as the SH-60K.

At least two members of the class, JDS Ōsumi (LST-4001) and JDS Kunisaki (LST-4003), participated in search & rescue and other disaster relief operations in the aftermath of the 2011 Tōhoku earthquake and tsunami.

As of 2013 there are 3 Ōsumi vessels active with the Japanese Maritime Self-Defence Force.