Jenson Button says he hopes to stay at McLaren beyond 2015.
McLaren said Button had signed a two-year contract. This is for 2015 with an option for 2016, BBC Sport has learned.
Button said: "To be part of McLaren-Honda for more than one year is important. I am not just here to be around for one year. I am here to fight for world championships."
The 34-year-old had previously expressed his frustration at the delays in resolving his future and said he did not want to comment on that again.
But Button said he was looking forward to taking on Spaniard Alonso, who many observers regard as the finest driver in the world.
"It's very special to be sat up here as a race driver with McLaren-Honda and alongside this guy," Button said, sitting alongside Alonso at the McLaren Technology Centre.
"It has been a long few weeks but sometimes the best things in life are worth waiting for. It is my 16th year in F1 and more importantly my sixth year at McLaren. I'm not one to shy away from new challenges and this is a big one.
"To race alongside this guy is a big challenge but an exciting challenge."
McLaren chairman Ron Dennis said that he had taken his time over deciding between Button and 2014 team-mate Kevin Magnussen because he had initially had some "causes of concern" about Button.
"I gave Jenson some ideas on solutions," said Dennis, without going into detail," and he said: 'No problem.' Then it was very easy.
|McLaren's 2015 partnership|
|Fernando Alonso||Jenson Button|
"We settled all the things between us that concerned me and what we all have to do is deliver against the commitments and promises we have made to each other."
Button said: "I have been very open in how I've felt. I don't think there is any reason to comment any further, the positive is where we are now. And we move on from here."
Magnussen has been retained as the team's third, reserve and test driver.
The 22-year-old Dane said: "It has been difficult to take. I believe in myself and that I can do great things, and I wanted to do that.
"My career has been paused - as Ron said - and that's not a nice thing but I still believe I have a future in F1 as a racing driver.
"Obviously I'm disappointed not to be racing. I want to race but I am still with a great team and I still have big opportunities in my F1 career. I am young and have a lot of time ahead so I'm still very positive."
Dennis drew parallels with the experiences of Alonso and McLaren's 1998-99 champion Mika Hakkinen, both of whom returned to racing after a year as a test driver early in their careers.