The international ratings agency lowered its view of the company to Ba1 from Baa3, meaning its debt is now seen as below investment grade, a move that threatens to push up Sony's borrowing costs.
"While Sony has made progress in its restructuring and benefits from continued profitability in several of its business segments, it still faces challenges to improve and stabilise its overall profitability," Moody's said in a statement.
"Of primary concern are the challenges facing the company's TV and PC businesses, both of which face intense global competition, rapid changes in technology, and product obsolescence."
It also pointed to a weaker-than-expected performance in its film business because of box office flops such as "White House Down" and "After Earth".
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The maker of Bravia televisions and PlayStation games consoles cut its full-year profit outlook to 30 billion yen from 50 billion yen.
Today's downgrade was the latest to hit Japan's embattled electronics industry which has continued to lose ground to overseas rivals even as a weak yen helped boost profitability.
Apple and South Korea's Samsung surged ahead of their Japanese rivals in the lucrative smartphone sector, although Sony has done better than its domestic rivals with its Xperia offering.
"Sony's profitability is likely to remain weak and volatile, as we expect the majority of its core consumer electronics businesses -- such as TVs, mobile, digital cameras and personal computers -- to continue to face significant downward earnings pressure," Moody's said.