Czech Republic midfielder Tomas Rosicky joined Arsenal from Borussia Dortmund on Tuesday.
The 25-year-old has signed a long-term contract for an undisclosed fee, the Arsenal website reported without giving further details. Czech media reported Rosicky's deal was for four years.
The move is a second boost to the London club following their 2-1 defeat by Barcelona in the Champions League final last Wednesday. On Friday France striker and club captain Thierry Henry decided to stay.
"I'm really happy to be here at Arsenal, it is a great day for me," said Rosicky.
"The team showed its strength recently in the Champions League and it has big potential with a lot of young players, I think we can be even better next season. This is the best club in Europe and I will give everything for Arsenal."
The attacking midfielder was contracted to Dortmund until 2008 and is a key member of the Czech Republic's World Cup squad. He has been a long-standing Atletico Madrid target.
Czech media reported that Rosicky flew to London on Monday and had passed his medical.
Arsenal manager Arsene Wenger said: "We are delighted to welcome Tomas to our squad. He has great technique, skill on the ball and sharp passing.
"Tomas has good experience at all levels for club and country, and we look forward to welcoming him to Arsenal after what will hopefully be a successful summer for him at the World Cup."
Former Sparta Prague player Rosicky made his international debut aged 17 and became the youngest winner of the Czech player of the year award in 2001 aged 20.
He joined Dortmund in 2001 and quickly became a crowd favourite, not least because the club pipped fierce rivals Bayern Munich to his signature. His sale will also go some way to easing Dortmund's financial problems.
