The financial agency, Standard and Poor's, has downgraded the rating of the Cook Islands to junk territory.
The agency cites a lack of positive information that could hold up the Cooks' credit status.
It now rates the Cook Islands at B-plus, four steps below investment-grade territory.
The downgrade comes after Standard and Poor's established new criteria for rating sovereign governments with limited external data.
The Wall Street Journal quotes analyst Kyran Curry as saying the Cooks external position is based on the assessment of the weak external score for New Zealand, whose currency the Cook Islands uses.
The outlook on both its local and foreign currency ratings is stable.