Japan's Ministry of Finance released data on Monday showing that foreign exchange reserves fell to $1.208 trillion at the end of August, down sharply by $79.6 billion from $1.287 trillion at the end of July—a single-month decline of 6.18%, with both the amount and the percentage drop marking the largest on record. The figures reveal the price Tokyo paid to prop up the yen and offer the first quantitative glimpse into the scale of its intervention.
The core driver behind the shrinking reserves was a substantial decline in foreign securities holdings. Data showed Japan's overseas securities positions fell by $87.8 billion month-on-month at the end of August, a magnitude closely aligned with the authorities' recent yen-supporting intervention. Japan's Ministry of Finance had previously confirmed that between July 30 and August 26, it deployed ¥15.4 trillion (approximately $98.7 billion) to buy yen and sell dollars—the largest single-month intervention on record.
Foreign securities account for roughly 70% of Japan's FX reserves, and market participants estimate the vast majority consists of U.S. Treasuries. Much of this Treasury stockpile originated from Japan's yen-selling, dollar-buying interventions roughly two decades ago. Now Tokyo is reversing course, selling dollar-denominated assets to fund yen support. Notably, the FX reserve data does not provide a breakdown of securities holdings or maturity schedules. However, with 10-year Treasury prices only slightly lower at the end of August compared to the end of July, valuation changes account for only a small fraction of the decline in overseas securities holdings—further evidence that the reduction stemmed primarily from actual sales rather than market price declines.
The intervention campaign briefly pushed the yen from around ¥164 per dollar, a 40-year low, to ¥155.2 on August 3. The yen subsequently weakened back toward ¥160 before recovering to the ¥155–156 range in early September. In Asian trading on Monday, the dollar traded at ¥156.05 against the yen.
Some intervention operations were conducted jointly by Japan and the United States—the first coordinated intervention between the two countries since 2011, catching markets by surprise. To alleviate concerns about Japan's limited intervention firepower, officials in Tokyo and Washington indicated that Japan could tap the U.S. Federal Reserve's central bank dollar liquidity facility established during the pandemic, allowing it to access dollar funding without directly selling large volumes of U.S. Treasuries and thereby reducing pressure on FX reserves and the Treasury market from future interventions.
Ripple Effects on the Treasury Market
Japan's selling of U.S. Treasuries comes at a time when the U.S. government is increasingly sensitive to long-term yield movements. If Tokyo again turns to selling U.S. Treasuries to fund future interventions, it would signal that Japan remains willing to pursue this path—even as Washington prioritizes maintaining stability in the Treasury market.
The concern is well-founded. U.S. Treasury Secretary Scott Bessent is preparing to expand the government's purchases of long-dated bonds. The Treasury has already indicated it will double the size of its long-term securities buybacks to at least $4 billion per operation by November 4, aiming to curb long-term borrowing costs.
Meanwhile, market expectations for a Bank of Japan rate hike continue to build. The yen's recent strength has been partly driven by a repricing of the BOJ's policy trajectory. As of early September, markets had almost fully priced in a 25-basis-point rate hike at the BOJ's September 17–18 meeting.
Intervention Effectiveness and Room for Further Action
The data suggests Japan's intervention did stabilize the currency in the short term. The yen rebounded from its 40-year low of ¥164 to the ¥155–156 range, averting a more severe depreciation spiral. However, after briefly strengthening to ¥155.2 in early August, the yen weakened back toward ¥160, underscoring that intervention alone cannot overcome the fundamental interest-rate differential unless the BOJ's monetary policy stance undergoes a substantive shift.
Market analysts note that even after the largest monthly decline in history, Japan still has ample resources for renewed intervention if needed. With $1.208 trillion in FX reserves, plus the backstop of the Fed's dollar liquidity facility, Tokyo is unlikely to face a depletion of intervention ammunition in the near term.
That said, sustained large-scale selling of U.S. Treasuries is not without costs. Japan is one of the largest foreign holders of U.S. government debt, and changes in its holdings carry outsized influence over supply-demand dynamics in the Treasury market. If Japan is forced to frequently sell Treasuries to fund interventions, it could push up long-term U.S. yields, raising borrowing costs for the U.S. government—an outcome neither country wants.
Note: FX reserve data is denominated in U.S. dollars; overseas securities holdings changes are month-on-month comparisons. The decline in total FX reserves is the largest on record.
From a longer-term perspective, Japan's record intervention reflects the structural weakness of the yen amid persistently wide U.S.-Japan interest rate differentials. As long as the Federal Reserve maintains relatively high rates while the BOJ hikes at a comparatively slow pace, the yen will struggle to escape depreciation pressure. Intervention can buy time and curb speculative selling, but it cannot substitute for fundamental monetary policy adjustments.
Market attention now focuses on two key dates: the BOJ's September 17–18 policy meeting, where the rate decision will directly influence yen movements, and the execution of the U.S. Treasury's expanded long-dated bond buybacks, which will test the market's capacity to absorb supply with reduced Japanese demand. The interplay between these two factors could amplify volatility in both the yen and Treasury yields.
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