The Argentina star arrives on the back of five goal involvements in just eight Serie A starts this season for Lazio, who agreed to let the striker go when the Hammers came knocking.
West Ham announced his arrival on Monday afternoon in a signing that will delight fans who have become tired of their toothless attack this season, which has brought the side 21 goals in 20 Premier League games this season.
Speaking to the club website, Castellanos spoke on the challenge ahead and who he will draw inspiration from as he takes on the Premier League.
“Good, I’m happy with the welcome, it's definitely a challenge. I'm really happy because it's a very important challenge for me personally, and I've come to contribute, to try to help the team as much as I can, and I'm very happy.
“Yes, obviously we know that every match is a battle, and I'm here to contribute that, to try to bring that energy, that fighting spirit I have inside, so that every match is as important and as tough as possible. And well, that's it, I'm going to try to give it my all on the field, which is the most important thing.
“Well, obviously it was a dream of mine since I was a kid to be able to play in this league. I always watched it when I was little, I saw many Argentinian players like Carlos Tevez, Sergio Agüero, Pablo Zabaleta… many high-level players who have played in this league and many are still here, so, well, for me it was a dream to be able to be here.”
The striker also sent a message to fans who are starting to accept that the club are in a relegation battle this season under manager Nuno Espirito Santo.
"I hope to give my all to the fans. I've always defended the jersey of every team with the utmost responsibility, and I want to tell them that I'm going to give everything during these years of my contract here, to defend this jersey, and obviously, to achieve our goals day after day.
"That's the most important thing. And I also want to thank everyone for all the support they've shown me so far on social media. I hope to achieve those goals, and I hope to see you all soon on the field.”