The Dow surged to a record Wednesday as US stocks welcomed congressional testimony by Federal Reserve Chair Janet Yellen reiterating the pledge to gradual interest rate increases.

The Dow Jones Industrial Average gained 0.6 percent to 21,532.14.

The broad-based S&P 500 advanced 0.7 percent to 2,443.25, while the tech-rich Nasdaq Composite Index jumped 1.1 percent to 6,261.17.

Yellen, appearing in a twice-annual hearing on Capitol Hill, reaffirmed the US central bank's stance pledging gradual rate hikes, as long as the economic data remains solid. 

The Fed chair held to a "dovish tone in critical ways," said Mickey Levey, chief economist at Berenberg Capital Markets, who noted that Yellen's emphasis on low inflation left the door open to a pause in rate hikes.

Technology shares were especially strong, with Facebook winning 2.3 percent, Google-parent Alphabet 1.5 percent and Microsoft 1.7 percent.

Amazon gained 1.3 percent as it hailed a successful Amazon Prime shopping day on Wednesday, saying it attracted more new members to the subscription service than on any previous day.

Another prominent tech company, Twitter, climbed 3.3 percent as it named Ned Segal as chief financial officer. Segal has worked as senior vice president of finance for Intuit. 

Airline shares were lifted after American Airlines said second-quarter revenue per seat mile had risen five-to-six percent from the year-ago period, a better range than previously. 

American Airlines rose 4.2 percent, Delta Air Lines gained 2.2 percent and United Continental 4.7 percent.