Andre Dede Morgan Ayew has signed a three-and-a-half-year contract at the Liberty Stadium after Swansea agreed to an undisclosed fee with the Hammers.
The 28-year-old forward returns to Wales having left the Swans to join West Ham for £20.5 million almost 18 months ago.
Ayew is a proven Premier League player who finished as the Swans’ 12-goal leading scorer in his one previous season at the club.
He immediately settled into the English game, scoring in his first two Swans appearances, and was an influential part of the team which overcame a difficult first half of the season to finish in mid-table.
Ayew’s contribution – his 12 goals came in 35 appearances – persuaded West Ham to spend big to take him to East London in August 2016.
The versatile forward will wear the No. 19 shirt in his second stint as a Swan.