The reigning GP2 Champion replaces German driver Nico Hulkenberg, who was released from the squad at the end of this season.
Venezuelan-born Maldonado will drive at the former champion team alongside the most capped F1 driver in the field – Rubens Barricello.
Pastor said that it was an honour to drive for Williams F1.
“I feel very privileged that Williams has selected me as one of their race drivers,” he said.
“It is a great way to end what has already been an amazing year for me. I tested with the team in Abu Dhabi, but can't wait to start working with everyone properly to be in the best possible shape going into next year.
“I will be doing my best over the winter to prepare myself, and I know the team will be working hard to ensure a successful season.”
Team Principal Frank Williams said that he is looking forward to seeing what Maldonado can do in his cars.
“Pastor first caught my eye in 2007 when he drove a masterful race at Monaco (in GP2),” Williams said.
“Since then, he has regularly reminded us of his unquestionable talent with some skilful wins, particularly during this year's GP2 championship, taking an unsurpassed record of six successive victories.
“Of course, we already know Pastor from the time he spent with the team at the Abu Dhabi test, but we are very much looking forward to developing his talent over the winter and a fruitful season with him next year.”
Maldonado, 25, started his motorsport career in karts at the age of seven, his passion for the sport ignited by a father and uncles who all competed. Having won seven national karting titles, he moved into formula racing in 2003.
This year, Maldonado won the 2010 GP2 championship, becoming the only driver to have ever won six-successive feature races in a single season.
He completed nearly 2000km of F1 testing at the recent Abu Dhabi test days and will make his debut in the team's 2011 race car, the Williams Cosworth FW33, in February's winter test programme.
Q&A with Pastor Maldonado (supplied by Williams F1)
QUESTION: What started your career in motorsport?
Pastor Maldonado: Having competed themselves, my father and my uncle are very passionate about motorsport, so I inherited it from them. In my city of Maracay, there is a go kart circuit about five minutes from my home. When I was about three or four years old I said I wanted to race but I was too young, then when I reached the age of seven my father gave me a kart and we started from there. From that moment until now we have never stopped.
After karting in Venezuela, I came to Europe in 1998 to compete in international kart races, which was great for me to get experience racing outside my country. After consistently being at the top, I decided to move to Italian Formula Renault. I won the championship in my second year. We made the jump to GP2 in 2007 but I only did half a season as I had an injury. We came back in 2008 and finished fifth in the championship, just six points adrift of the leader in a very close championship.
Q: You were crowned GP2 champion this year. What does that feel like?
PM: It was an incredible season. We were competitive from the beginning and went on to win six races. The team worked well together to achieve victory and by the middle of the season I already had a good gap and took the title at Monza.
Q: Do you think you are ready for F1?
PM: GP2 is a very good championship; it really prepares drivers well for F1. I have worked very hard to get to this position and yes, I definitely feel ready.
Q: How does it feel knowing you will be driving for AT&T Williams next year?
PM: Williams do an amazing job. It is unbelievable to be here and to be part of the team. It is a dream.
Q: What do you make of your new teammate Rubens Barrichello?
PM: For sure Rubens is a pleasure to have as a teammate as he is a very experienced driver. I can learn so much from him. It will be fun as he is South American too! I think it is going to be a very interesting team.
Q: How will you prepare for your first F1 season over the winter?
PM: I will keep pushing in my training and working in the simulator. We don't have a long time, just one or two months before the first test, and I am going to be fit and ready.
Q: You had a day in the FW32 at the Abu Dhabi Young Driver test. How did that go?
PM: It was amazing. It was a big moment for me because only days before I had been driving a GP2 car and there are some big differences. I improved lap after lap and completed the program so it was a very good experience for me.
Q: What are the differences between a GP2 car and an F1 car?
PM: There are three big differences. The first is the difference in engine power; the F1 car has amazing power and a higher top speed. Secondly, the braking point; the brakes are a lot harder in F1. Finally there is much more downforce and general grip.
Q: You will be only the fourth Venezuelan to have ever driven a Formula One car. What does that mean to you?
PM: It has been nearly 30 years since Venezuela has had a driver in Formula One so the country has been pushing young drivers in the hope of having someone represent them. I am happy to now be that driver.
Q: To summarise, what will be your objectives for the 2011 season?
PM: I just want to do my best, to be as close to the top as I can and to get the maximum out of the car. The team are working very hard and I need to push to be at the top as soon as possible. I am a rookie but that isn't going to be a problem. I need to keep focused and to do my job.