The forward becomes the Hammers second signing of the day following Antonio Nocerino's arrival from AC Milan, with both players penning deals until the end of the season
Striker Borriello follows compatriot Antonio Nocerino to Upton Park after the midfielder made a similar switch from Milan earlier in the day.
Borriello joined Roma from Milan for £10 million in July 2011 following a successful loan spell, but has struggled to establish himself at the Stadio Olimpico.
The 31-year-old has spent time at Juventus and Genoa on loan, and scored 12 goals while at the latter for the duration of last season.
However, the forward, who has seven caps for Italy, has managed just one goal in 11 appearances for Roma this term.
West Ham boss Sam Allardyce will hope Borriello can recapture his best form alongside Andy Carroll up front as the Londoners, who have won just one of their last nine Premier League matches, battle to avoid relegation.
Borriello said goodbye to Roma fans on his official Twitter page on Friday, but left his destination out of the message.
"A salute to all the fans of Roma," he wrote. "These have been fantastic months, crowned by a wonderful record. Thank you all."