The Romanian captain arrived from Shakhtar Donetsk on a free transfer earlier this month after playing over 300 games for the Ukranian champions.
And Rat said he can't wait for the new season to come, insisting it will be a very proud day.
"First of all I am very happy to wear a West Ham United kit," Rat told West Ham TV.
"I think this kit has a very good quality and is of high standards.
"For me it's an honour to wear this shirt and to play in the Premier League. I will be very proud to wear the claret and blue colours and I hope to satisfy the fans.
"I'll be very excited when I pull on the kit for the first time, because it will be my first game here."
Rat has played 89 times for Romania, making his debut back in 2002.