The 26-year-old moves to The Hawthorns for an undisclosed fee and is the Midlands club's fourth arrival of the transfer window.
With 12 international caps to his name, Pocognoli missed out on a place in Belgium's FIFA World Cup squad after seeing his season curtailed by injury.
Albion are looking to improve on last season's disappointing season which saw them finish 17th in the Premier League after a season-long relegation scrap.
Former Everton assistant coach Alan Irvine was brought in to replace Pepe Mel and revealed ex-Albion loanee Romelu Lukaku was among those to have recommended Pocognoli to the club.
Irvine told the club's official website: "Sebastien is an experienced left-back who has played at a high level for many years.
"After being named in Belgium's provisional World Cup squad, he just missed out on Brazil, mainly because he hadn't played enough games towards the end of the season.
"But he's joined us with a real hunger to prove himself at Albion and get back into the international set-up.
"We've done our due diligence on Sebastien in terms of people who have worked with him, including Romelu Lukaku and Kevin Mirallas.
"You make your own mind up about a player's ability from watching him play but you put these calls in to find out about their personality. The feedback on Sebastien was very positive."
