An issue of debt securities that carries an interest rate very close to the prevailing market rates (in the secondary market), irrespective of the time it was introduced to the market. For example, a bellwether Treasury issue is the most recently auctioned Treasury issue for a given maturity. A bellwether bond is considered a standard (a benchmark) against which the performance of other bonds is related. Government bonds (of stable and credit-worthy economies) are often used as bellwether bonds, in view of their standard features and fundamentals.

Bellwether issues are usually used as the basis for constructing yield curves.

A bellwether issue is also known as a benchmark issue or an on-the-run issue or a current-coupon issue.