The 31-year-old left-back joins the club from Shaktar Donetsk, on a one-year contract with the option of an extension for another two or three years.
Hammers boss Sam Allardyce spoke in enthusiastic terms of Rat as he welcomed him to the club, “I’m hugely happy that we’ve got a player of his experience and his character.
“He’s been playing Champions League football this year and at the highest level for many years. Shakhtar had offered him a very good contract, but his ambition was to come and try his abilities and skills in England.
“When you hear somebody speak so positively about that, then you know they’ve got the mental character that’s needed to do well in this division.
“We certainly know he’s got the ability and he’s at an age where he’s still capable of producing at 31.
“He speaks perfect English, which he taught himself, and that shows the discipline and the character of the man. Discipline, character and ability are the three big things in the Premier League, and with a little bit of adjusting, we’ll see him early on in the season giving hopefully his best performances for West Ham.”