The hand-made red clay brick was found under the old promenade in front of Southsea Castle during coastal defence construction works.
It was part of an excavated structure built to house a swivel gun and dates back to the early 19th century.
Councillor Tom Coles, the Lord Mayor of Portsmouth, is delivering the brick to Japan personally during a visit to mark the 25th anniversary of Portsmouth becoming a sister city to Maizuru and to commemorate the 80th anniversary of Maizuru becoming a city.
"It's a privilege to donate this brick to our sister city as part of our 25-year commemoration,” said Coles.
"The brick will sit alongside worldwide examples of red brick, including one from the historic dockyard in Portsmouth which was donated in 1993.
"The fact that the brick is likely of military origin and discovered during coastal works makes it even more appropriate to Maizuru, given both of our cities' coastal locations and rich naval and military history."
Maizuru is famous in Japan for its abundance of red bricks which can be seen in buildings across the city. Its World Brick Museum was built from red brick by the former Navy in 1903 and was originally used as a torpedo warehouse.
The museum's website declares that “inside this museum you will find bricks from all over the world, hence the name the World Brick Museum. On display are bricks from the four great civilizations of the world, ancient Mesopotamia, Egypt, China, and Greece”.