The article analyzes the current stance of the Republic of Korea (ROK) and Japan on the institutionalization of defense cooperation in the Asia-Pacific region, provides the assessment of the prospects for the US-ROK-Japan triangle institutionalization, as well as the assessment of the prospects for the possible accession of Japan and South Korea to the AUKUS agreement. Drawing from the strategic documents of Tokyo and Seoul in the field of security, as well as joint statements and other official information materials of the United States and its partners in the Asia-Pacific region, the author comes to the conclusion that the announced "Trilateral Security Cooperation Framework" of the United States – South Korea – Japan will be poorly formalized, and the main area of cooperation within this framework will be the exchange of intelligence data and monitoring of the military activity of the DPRK. The Memorandum of Cooperation concluded in July 2024 has primarily an ideological significance and is intended to create a precedent for formalizing military cooperation in a trilateral format. The potential entry of the ROK and Japan into AUKUS also has its problems. Through the license-free AUKUS arms trade ecosystem, Japan potentially can be able to develop its own defense industry and gain a foothold as an arms exporter. On the contrary, for South Korea, which is already an established exporter of military equipment and weapons, this would be a potential risk and the possibility of losing its acquired positions in Australia and other markets. Nevertheless, the prospect for both parties joining AUKUS is real, and cooperation within this multilateral format can also be of a weakly formalized nature, reduced to work on a number of joint projects, mainly in high-tech areas with the ultimate goal of developing common technological and legal standards.