US 3-month bill yield hits 4% as Treasury auction demand weakens

US Treasury bill yields just crossed a threshold that tends to get people’s attention. Both 3-month and 6-month bills are now offering returns above 4%, a level that hasn’t been the norm for these short-term instruments during the summer months, when yields were comfortably parked in the 3.7% to 3.9% range.

What the auction numbers show

The September 14 Treasury auction moved $92 billion in 13-week bills at a high rate of 4.066%. The 26-week bills cleared $79 billion at 4.203%.

The bid-to-cover ratios, which measure how many dollars of bids came in for each dollar of bills on offer, painted a less enthusiastic picture. The 3-month bid-to-cover came in at 2.64x, while the 6-month ratio landed at 2.74x. For context, 3-month bid-to-cover ratios dropped as low as 2.32x back in June 2026, and recent auctions have averaged around 2.6x.

By September 21, secondary market yields had pushed even higher: 4.084% for 3-month bills and 4.279% for 6-month bills.

The slow climb through September

Yields on 6-month bills in the secondary market crept from 3.89% on September 4 to 3.95% by September 11. The auction on September 14 then punched through 4%, and secondary market trading in the days after kept the momentum going.

The rise from sub-3.9% to above 4.2% on the 6-month bill over roughly three weeks represents a meaningful shift in the cost of short-term government borrowing. For a $79 billion issuance, even small yield changes translate into real dollars for taxpayers.

What this means for markets and portfolios

A 4%-plus return on a risk-free instrument that matures in three to six months creates a gravitational pull on capital. For institutional investors managing large fixed-income portfolios, longer-duration bonds become a harder sell when you can earn 4% on something that matures before the next earnings season.

Money market funds, which hold large quantities of Treasury bills, stand to benefit from the higher yields. Investors who parked cash in these funds during the rate-hiking cycle may find even less reason to redeploy into riskier assets now that returns are climbing again.

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