
The 30-year-old is set to be unveiled at a press conference at 0100 GMT, joins Craig Bellamy, Scott Parker and Frenchman Julien Faubert in moving to Upton Park this summer.
Ljungberg joined Arsenal in 1998 and scored 72 goals in 325 games for the club, helping the London side to the League and Cup double in 2002.
He had two years left on his contract but struggled for fitness last season with hamstring trouble.
DPA