About 15 minutes into trading, the Dow Jones Industrial Average had gained 0.5 percent to 24,844.42.
The broad-based S&P 500 also rose 0.5 percent to 2,686.70, while the tech-rich Nasdaq Composite Index jumped 0.7 percent to 6,948.82.
After a wave of all-time record highs through much of last year, US stocks closed 2017 in muted fashion, dipping in the final session of the year Friday in low-volume trading.
Most analysts expect stocks to rise further in 2018 amid strengthening global economic conditions and higher US earnings expected due to tax cuts. But analysts also expect greater volatility compared with the placid 2017.
Key data releases this week include December auto sales on Wednesday and the December jobs report on Friday.
Among notable stocks, Disney climbed 1.3 percent on more positive news about "Star Wars: The Last Jedi," which already topped $1 billion in world ticket sales after just three weeks and was ranked the top movie at the US box office for 2017.
Technology companies with sizeable gains included Facebook, up 1.4 percent, Expedia, up 1.0 percent and Netflix, up 3.1 percent.
Big-box retailer Costco Wholesale gained 2.0 percent and agricultural products retailer Tractor Supply won 1.8 percent.