The 19-year-old, famous for her role as strait-laced Hermione Granger, kept on smiling as an ungodly downpour struck at the exact moment she arrived on the red carpet.
The actress is certainly growing up quickly and she seemed to enjoyed the chance to try a bold new fashion look in a free-flowing vintage 1970s Ozzi Clark dress.
All grown up: Emma Watson leaving London's Middle Temple last night after the premiere of Harry Potter And The Half-Blood Prince
Emma led her Harry Potter co-stars, including Daniel Radcliffe and Rupert Grint, at the highly anticipated world premiere of the sixth film.Radcliffe and Grint managed to keep their sodden look for the next 45 minutes with their spirits also apparently buoyed by the screaming audience in the Leicester Square.
After drying off, Watson later went on to celebrate with boyfriend Jay Barrymore at the after party held at Middle Temple.
All the cast members and crew arrived wearing white ribbons in memory of Rob Knox a young actor on the film who was stabbed to death in London last year.
Not just your average muggle: Emma arrived at London's Leicester Square just as the heavens opened up
The latest film is the penultimate in the series based on the books by JK Rowling and cost £150million to produce.
However, the fifth instalment Harry Potter And The Order of the Phoenix, earned more than £700million at the box office globally and this movie film set to be equally successful.
Half Blood Prince, released on July 15, as been dubbed the darkest film of the series yet. Fans will see apocalyptic scenes as Lord Voldermort and his followers (known as Death Eaters) cast their evil spell on the muggle (human) world.
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Drenched: Daniel Radcliffe, who stars as Harry Potter, got drenched as he stepped out onto the red carpet
Singing in the rain: Rupert Grint didn't let the bad weather dampen his spirits
Downpour: Rupert Grint (Ron Weasley) and Tom Felton (Draco Malfoy) swapped their broomsticks for a brolly
Trio of wizards: Grint, Watson and Radcliffe pose together after managing to take shelter from the rain
Harry, 'the Chosen One', also goes on a journey of discovery as he prepares for his final showdown with Voldermort.There are lighter moments in the movie too, as Hermione and Ron share their first kiss and Harry and Ginny Weasley develop a crush on each other.
Alan Rickman reprises his role as Snape, Dame Maggie Smith is back as Professor Minerva McGonagall and Sir Michael Gambon returns as Albus Dumbledore.
Wand-erful: Harry Potter author J.K. Rowling arrived in a dazzling purple dress and, right, Helena Bonham Carter, who plays evil Bellatrix Lestrange, wore a girly ensemble
Touch of Klass: Myleene Klass, singer Jamelia and Danielle Bux were among celebrities who turned out to the Odeon cinema
Deathly Hallows, the final book in the series, has been divided into two epic films. Part I will be released in November of 2010, with Part II set for release in July of 2011.At the premier last night the young stars greeted the thousands of fans shaking hands and signing autographs for half-an-hour.
Many of the gathered crowd had been in Leicester Square since early Monday desperate to get close enough to see the stars, some even travelling from across the globe to be there.
Fun for the family: Jamie and Jools Oliver with two of their daughters, Poppy and Daisy (L) and Gordon Ramsay and wife Tana with their three children
Casting their spell: Evanna Lynch (Luna Lovegood), Bonnie Wright (Ginny Weasley) and Dame Maggie Smith (Professor McGonagall)
Another Dominiq Meyer, 21, also from Brazil, said: 'Harry Potter for us is not just a movie. We are such huge fans and we wanted to be here, it's a huge moment.'
Sian Windmill, 16, a student from Bedford, spent seven hours waiting outside the cinema. She said: 'I'm not planning on eating or drinking because I can't get out of the crowd.
'We've come to see Ron because he's the best character. We've never been at the front before.'
The cast of the film were joined by Miss Rowling who herself who earned a rapturous reception as she entered the Square.
For the occasion she dressed in an Alberta Ferretti custom-made purple silk and chiffon dress teamed with gold heels.
Top form: Robbie Coltrane (Hagrid) Alan Rickman (Professor Snape) and Miriam Margolyes (Professor Sprout)
Dedicated: Fans line up and take shelter as they wait for the stars to arrive
Mass of muggles: A 3,000-strong crowd turned out to the premiere last night, braving the torrential rain
A galaxy of other stars put in an appearance too, including Jamie and Jools Oliver and two of their daughters and Guy Ritchie and his son Rocco.Myleene Klass turned on the glamour in a flattering white knee-length dress, while singer Jamelia opted for the leggy look in a revealing pink and red coloured mini dress.
Commenting on the crowd Emma Watson said: 'It's just so fantastic that people have been so dedicated to come down and see us all. My message to the fans is thank you so much I love you all.'
Watson said she'll be sad when the series finally comes to an end in 2011.
'We'll all miss it,' she said, 'but I think you underestimate the longevity of the books and the series. I don't think it's going anywhere fast, I think new generations of children will keep reading the books and hopefully watching the films.'
The long goodbye: JK Rowling with Rupert, Emma and Daniel, who have almost filming the last installment
