The Lisbon stock exchange rose more than 1% on Monday and the 10-year bond yield dropped 0.29 percentage points - an unusually large one-day increase - to 2.49%, the lowest rate since late 2015.
S&P said on Friday that Portugal has made "solid progress" in restoring its financial health since needing a 78 billion euro (£68 billion) bailout in 2011 amid the eurozone crisis.
It raised its rating from BB+, or junk status, to BBB-.
Fitch and Moody's, the other two main ratings agencies, still rate Portugal's debt at junk.
After a period of austerity and recession, Portugal expects the economy to grow 2.5% this year.