Thailand hopes that the agreement facilitating the transfer of defence hardware and technology will help improve its national defence and support investment from Japan.
The leaders of Japan and Thailand have announced a new defence agreement as well as plans to upgrade their economic relations.
Monday's announcements came as Japanese Prime Minister Fumio Kishida wrapped up the last leg of a three-nation tour of Southeast Asia.
The agreement would facilitate the transfer of defence hardware and technology from Japan to Thailand, which has one of the region's biggest armies and a long history of US military ties.
"This will help improve national defence and support investment from Japan in this activity which is an important goal for Thailand," Thai Prime Minister Prayuth Chan-ocha said in a joint statement.
Further details of the deal were not disclosed.
Prayuth said he discussed improvements in supply chains and the drafting of a five-year economic partnership with Japan, Thailand's biggest investor.
Southeast Asia has for decades been an important region for Japan, hosting some of its biggest names in industry, from infrastructure, engineering and industrial zones to the manufacturing of vehicles and electronics.
The region remains a battleground between the United States, Japan's close ally, and rival China, Southeast Asia's biggest trade partner.
On his three-day trip Kishida also visited Vietnam and Indonesia, where Japanese firms maintain a large presence.
As the leader of Asia's sole member of the Group of Seven (G7), Kishida discussed Russia's attacks on Ukraine during his trip to Southeast Asia, where only one nation—Singapore—has joined sanctions against Moscow.
Russia calls its actions in Ukraine a "special operation".
Nine Southeast Asian countries backed a United Nations resolution in March condemning the Russian offensive, however, and Kishida thanked Prayuth for Thailand's support.
"I agreed with Prime Minister Prayuth that in any region the violation of sovereignty and territorial integrity, or unilateral changes to the status quo with force, should not be tolerated," he said.
Noriyuki Shikata, Japan's Cabinet Secretary for Public Affairs, earlier on Monday told reporters that Tokyo would be extending a $385 million loan to support Thailand's Covid-19 mitigation efforts.