That's why tongues wagged this week when the Factory Girl actress was seen on three occasions with actor Rhys Ifans.
But we can reveal that the pair are not an item - the Welshman has been crashing on Sienna's sofa since they returned from holidaying on the party island of Ibiza.
Apparently his houseboat, which is moored in north London, has flooded, and according to sources, he only has eyes for model Iris Palmer, whom he's been dating for the past few months.
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'Sleeparound Sienna' Miller with good friend Rhys Ifans who is currently crashing on her sofa after his houseboat was flooded
Being the good housemate: Rhys Ifans takes Sienna's two dogs, Porgy and Bess, for a walk to a market in west London
The model girlfriend: Iris Palmer
The 25-year-old beauty exclusively revealed: "Rhys and I are just friends, we've known each other for years, we are definitely not sleeping together.
"He's crashing on my couch because his boat is flooded and I'm being the good samaritan and providing him with a roof over his head."
Sienna fuelled rumours that Rhys was her latest squeeze after they jetted off to Ibiza together last week.
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Just good friends: Miller and Ifans looked very cosy on their recent jaunt in Ibiza
When they returned to London, many presumed Rhys, 40, was romancing the boho blonde after they were spotted together in a taxi on Tuesday night.
The following morning they were seen again near Sienna's Notting Hill home.
But, in our exclusive chat with the actress, she spoke of her annoyance at being accused of being romantically involved with every male she meets.
She said: "I am always hearing things about myself and different men, but it's not true.
"Is my nickname Sleeparound Sienna or something?
"It's terrible, I can assure you I don't do anything of the sort.
"I am a single girl who likes to enjoy herself but it does hurt me when people say I am sleeping with every man I'm seen out with."
The pair made little effort to hide their obvious affection for one another during their trip to the beach
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