The 79-year-old woman was driving along the B3072 Verwood road, near Three Legged Cross, when she collided with two cars travelling the other way.
The other drivers, an 18-year-old man from Three Legged Cross and a 27-year-old man from Poole, were uninjured in Wednesday night's crash.
A man was arrested at the scene and later released on bail. Police are appealing for witnesses.
A Dorset Police spokesman said they want to trace the driver of a white family-size car, who could have vital information about the crash.
He said the pensioner died at the scene after her Vauxhall Cavalier first collided with a blue Honda Civic and then crashed into a BMW, which had been following the Honda.
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