The Swiss blue-chip index SMI, a measure of the largest and most actively traded companies, rose 0.58%, or 34.90 points, to 6,031.24. The broader Swiss Performance Index gained 0.54%, or 29.13 points, to 5,440.51.
"Part of the rating agency's action was expected by market participants. Therefore it didn't throw big waves," said Christoph Riniker, head of strategy research at Julius Baer Group Ltd. in Zurich.
"The reputation of rating agencies has recently suffered somewhat, which could also be a sign that downgrades aren't translated one-to-one in the markets any more."