Foreign investors sold a record ¥1.28 trillion of short and medium term JGBs in July, the biggest monthly outflow since July 2006.
At the same time, they bought ¥889.8 billion of bonds with maturities of 10 years or longer.
The selling came as the yen weakened sharply and markets increased bets on an earlier BOJ rate hike.
The BOJ kept rates at 1% on July 31, but Governor Kazuo Ueda signaled that a September hike could be considered. Markets are now pricing roughly an 80% chance of a September hike.
The key shift: investors are cutting shorter maturity JGB exposure while adding long duration bonds as expectations for faster BOJ tightening rise.