The defender completed a move from Olympiakos on Monday.
The full-back completed a move to the Olympic Stadium on Monday, with Slaven Bilic paying Olympiakos £6.2 million for his services, as reported by The Daily Mail.
He has been brought in to fill the currently vacant left-back spot after Aaron Cresswell picked up a long-term injury in pre-season.
West Ham's Aaron Cresswell in action with Leroy Fer
The Premier League is likely to be a big step-up for the full-back compared to the Greek Superleague, and he has admitted that he will have to work on the defensive side of his game.
“I think I am a little bit fast with the ball and I have a good left foot,” he told the club’s official website.
“Of course I have to work on my right foot, but I have a good left foot. I need to work on my defence, but I think in attack I am quite good. I have to work to be focused also on my defence.”
Olympiacos' Arthur Masuaku and Dennis Praet in action
It now remains to be seen if Masuaku is the only arrival at the club this week as they prepare for the start of the Premier League season.
Slaven Bilic said last week, as reported by The Express, that he was hoping to have five new players in before they kicked off, given the fact they have a busy month ahead of them.
The West Ham manager has already secured one in Masuaku, and it will be interesting to see if more arrive before the weekend.
West Ham manager Slaven Bilic