The Romney camp earlier said the Republican hopeful would announce his vice presidential pick around 9am (13:00 GMT) in Norfolk, Virginia during a prime photo opportunity at the USS Wisconsin, a decommissioned World War II-era battleship that also saw service during Operation Desert Storm in 1991.
NBC News, CNN and The Huffington Post were among US media that cited at least two campaign sources confirming Ryan would get the nod.
The announcement venue on its own made Ryan a likely pick.
Ryan, a wonkish budget hawk whose plan to slash federal spending and lower taxes has won over many Republicans, is from the state of Wisconsin, which shares the name of the battleship.
The timing of the roll-out means Romney will have his running mate by his side throughout a vital tour bus swing through the battleground states of Virginia, North Carolina, Florida and Ohio, four states that Obama won in 2008 but are in play this time around.
If Romney does announce Ryan as his choice, it would mean he is gambling that voters want nothing less than a showdown over the nation's fiscal future and the role of government in American life.
A serious and impassioned yet even-tempered fiscal pitbull, Ryan has emerged as a star in the House of Representatives' Republican caucus, which has embraced his plan to slash spending.
Ryan, 42, has been praised in recent weeks in several prominent editorial pages.
In an editorial on Thursday, The Wall Street Journal said the conservative darling would be a strong choice for Romney that "best exemplifies the nature and stakes of this election," with the sour economy high on top of voters' concerns.
"More than any other politician, the House Budget Chairman has defined those stakes well as a generational choice about the role of government and whether America will once again become a growth economy or sink into interest-group dominated decline," it said.
Unveiling the vice presidential pick at the USS Wisconsin is sure to carry heavy military symbolism at a time when the Pentagon is facing drastic budget cuts.
Romney has criticised President Barack Obama's administration for slashing hundreds of billions of dollars from military programs. Sapa-AFP