The downgrade will see Noble's senior unsecured bond ratings drop to Ba1, from the lowest investment grade of Baa3. The outlook for the rating is negative.
The cut to junk territory will likely increase Noble's borrowing costs and make it harder for the trader to refinance debt to shore up its finances.
Moody's sounded a warning on Noble's rating in November by announcing a review of the company's investment grading.
Noble's shares tanked as much as 5.7 percent in early trade Wednesday on the downgrade.