Police are appealing for witnesses to the smash, which took place in Loddon Bridge Road at just before 8.30pm yesterday.
The woman, said to be in her sixties, was in collision with a black Citroen C1 hatchback.
She was was taken by ambulance to the Royal Berkshire Hospital with head and neck injuries and was then transferred to the John Radcliffe Hospital in Oxford.
Police say she remains in a serious, but not believed to be life-threatening condition.
The driver of the car was unhurt.
Officer closed the road for around two hours while they investigated the circumstances of the collision.
PC Mark Brazier said: “I am appealing for anyone who witnessed the collision to contact police.
“I would also be interested in speaking to anyone who witnessed the vehicle prior to the collision.”
Witnesses should call the police on 101.
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