The Switzerland midfielder is set to arrive from Hamburg in the coming few days.
The Swiss international spent two and a half seasons with the Hammers after signing from Lazio in 2008. He played 62 games at Upton Park, scoring 5 goals, where his versatility made him a strong squad member, before departing for Fiorentina in 2011.
Behrami, who has appeared at three World Cups including last year's spectacle in Brazil, only moved to Hamburg last summer, but was part of a side that badly struggled in the league, only saving themselves from relegation late on in their playoff with Karlsruher. He played 23 times, almost exclusively in defensive midfield, but was unable to score or assist a goal last season.
GianlucaDiMarzio.com have said the 30-year-old is set to head to London on Friday for a medical, and could end up signing a three-year contract at the Hornets that would continue to strengthen the Premier League newcomer's midfield.