Film studio Warner Bros is looking for a girl between the ages of eight and 12 to play the character of Modesty.
"Modesty is a haunted young girl with an inner strength and stillness," the studio said. "She has an ability to see deep into people and understand them."
The casting call will take place at the Excel Centre in London on Saturday.
The chosen youngster will star opposite Eddie Redmayne in what is bound to be one of the biggest film releases of next year.
Rowling has written the screenplay for the film, which is based on a Harry Potter spin-off book she published in 2001.
Redmayne will play "magizoologist" Newt Scamander, who writes a Hogwarts School textbook about the weird and wonderful fictional creatures he encounters.
Hopeful actresses have been invited to start queuing from 09:00 BST on Saturday.
Fantastic Beasts and Where to Find Them is expected to be released on 18 November 2016.
Writing on her Facebook page in 2013, JK Rowling said: "Although it will be set in the worldwide community of witches and wizards where I was so happy for 17 years, Fantastic Beasts and Where to Find Them is neither a prequel nor a sequel to the Harry Potter series, but an extension of the wizarding world.
"The laws and customs of the hidden magical society will be familiar to anyone who has read the Harry Potter books or seen the films, but Newt's story will start in New York, 70 years before Harry's gets under way."

The 113, including children and pregnant women, landed in Nuevo Laredo on Tuesday.
They were among thousands stranded in Costa Rica after neighbouring Nicaragua denied them passage north.
Cubans are allowed automatic residence when they set foot in the US.
But after the two counties normalised ties following years of animosity, the number of Cubans trying to reach the US surged amid fears the policy would be dropped.
Central American nations reached a deal last month allowing migrants stranded in Costa Rica to continue on to the US.
The first flight carrying about 180 flew from Costa Rica to El Salvador in January.
An open casting call is to be held to find a girl to star in JK Rowling's Harry Potter spin-off film Fantastic Beasts and Where to Find Them.
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A group of vulnerable Cuban migrants have been flown by the Costa Rican authorities to a Mexican border town, from where they are expected to enter the US.