The Benin international has moved to The Hawthorns in a record-breaking deal for the Baggies that sees the attacking midfielder become their most expensive player in history.
The 29-year-old arrives at the club after two full seasons at the Stadium of Light, signing for an undisclosed transfer fee -- believed to be in the 6 million pound range -- that surpasses the deal that brought Shane Long to the club in 2011.
Sessegnon had initially dismissed speculation of a move away from the club in July by confirming he was happy to stay in Paolo Di Canio's new-look Sunderland side. The former Paris Saint-Germain player has now swtiched colors as Steve Clarke looks to make final alterations to a side that has failed to score a single goal in the opening three games of the league season.
The moves comes as his former side announced a deal to bring Liverpool striker Fabio Borini and Napoli defender Andrea Dossena to the club on season-long loans, taking their summer dealings to 14 in what has been a radical overhaul of the squad.