Sienna Miller looked effortlessly chic in a black co-ord as she opened The Treasure House Fair at Royal Hospital in Chelsea, London, on Thursday.

The Hollywood star, 42, flashed a glimpse of her midriff in a classy crop top with puffed sleeves and a matching skirt.

The Burn star - who gave birth to her second child five months ago, looked radiant and displayed her natural looks with a barely visible make-up.

Sienna added floral-shaped maxi pendant earrings giving a touch of colour to her ensemble, beaming as she posed at the event and later on took to the stage for a speech.

The fair, which takes place from 27 June through 2 July, will bring together 70 leading galleries and will feature over 20 collecting categories in the fields of fine art, furniture, antiquities and jewellery. 

Sienna Miller looked effortlessly chic in a black co-ord as she opened The Treasure House Fair at Royal Hospital in Chelsea, London, on Thursday

Earlier on Tuesday, Sienna appeared on Jimmy Kimmel Live! which was taken over by Martin Short, kicking off Jimmy Kimmel's summer hiatus as there will be a rotation of guest hosts filling his spot.

Martin was joined by his pal Steve Martin and the comedians warmly greeted Sienna before she hilariously explained she had already met them.

She recalled: 'It's funny that you say lovely to meet you, we've actually met. I had this horrible experience. Do you remember Dimitri at the Sunset Tower, the sweetest man, the greatest man in the world?

'I think he had a tactic of thinking there's a somewhat famous person and two very famous people and I want them to meet.

'So he came up to me and said, "Marty Short and Steve Martin want to me you". And I died a thousand deaths obviously, you know icons!

'So I come up to your table and I'm beside myself and I say, "Hi" and I stand there... And you were both like, who are you? You crazy person!'

Sienna also spoke about her new film Horizon: An American Epic: Chapter 1 directed, co-written and produced by, as well as starring Kevin Costner.

The film is the first of four chronicling a multi-faceted, 15-year span of pre-and post-Civil War expansion and settlement of the American West.

Kevin plays pioneer Hayes Ellison in the new film, who treks the West through dangerous territory in hopes of a new life.

While Sienna stars as Frances Kittredge in the period epic, which is set to be released in two chapters on June 28 and August 16 this year.

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They mark the first two of four planned film instalments for the movie series, which Kevin has been desperately trying to bring to life for more than 30 years now.

The Dancing With Wolves star, 69, is thought to have personally raised up to $50 million of Horizon's $100 million budget.

Describing how she came onboard the project, Sienna told Martin and Steve: 'I got this call from Kevin Costner, which exciting enough because I am a child of the 90s.

'And I get four scripts on my desk - the desk breaks because they're incredibly dense and heavy. And we had this chat and I'm so excited because he's Kevin Costner and he's a legend.

'And at the end of it he says, "Sienna I have one question for you. Will you go West with me?" I said I would go to Mars and back.'

Her TV interview comes after Sienna gushed over her 'wise' and 'well-adjusted' boyfriend Oli Green and said his 'generation of men' are more respectful to women.

The couple were first linked in February 2022 and welcomed a baby daughter together five months ago.

Making rare comments about her relationship in an interview this month, she lauded Oli, 27, as being 'wise and well-adjusted' and said his generation of men respect women more.

Sienna famously faced public humiliation in 2005 when her then-boyfriend Jude Law, now 51, confessed to cheating on her with his children's nanny Daisy Wright.

Speaking about Oli to Harpers Bazaar magazine, she said: 'I didn't expect to take it seriously and then quite quickly, I fell in love.

'I wasn't like, "I'm gonna get a younger boyfriend." It was more, "F*ck! Why are you young? That's so annoying."

'There is a difference in the way that generation of men respect women. It's specific to him, he is very wise and well-adjusted, but I do believe it's also that generation.

'They have grown up with a slightly more level playing field. I see it in his female friends as well as in the men.'

As well as the five-month old she shares with Oli, she has daughter Marlowe, 11, with her ex partner Tom Sturridge.