Markit said the Purchasing Managers Index for non-manufacturing businesses indicated a "robust expansion of U.S. service sector activity."
The index, which shows expansion above 50, rose from December's 55.7 to 56.6, Markit said.
The numbers are considered a flash estimate that could be revised later. The estimate is based on 85 percent of the data that goes into the final estimate, Markit said.
Markit said the service sector's PMI was "comfortably above the average seen for 2013 as a whole -- 54.8.
The index for new orders for January came in at 56.4. The employment index measured 54.1 in the month.
"U.S. service providers reported a busy January, providing an important signal that the economy remains in good health," said Markit Chief Economist Chris Williamson in a statement.
Williamson said the flash index suggests economic growth of around 3.5 percent in the final quarter of 2013.