The 12-year-old will play Shireen, daughter of Stannis Baratheon (Stephen Dillane), according to various Twitter posts.
Ingram wrote on her own personal account: "Can't wait to start on Game of Thrones."
Her casting and role were later confirmed by the Berkshire Arts Academy, who tweeted: "Well done [Kerry] for getting the role of 'Stannis Baretheon' daughter 'Shireen' in 'Game Of Thrones S3'."
A list of new characters debuting in the third season of Game of Thrones was recently unveiled, confirming the presence of both Shireen Baratheon and the character's mother Lady Selyse Florent.
Ingram recently won an Olivier Award for her role in the musical Matilda. She will next be seen in the film adaptation of classic musical Les Misérables.
The third season of Game of Thrones will air on HBO in the US and Sky Atlantic in the UK in early 2013.
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Watch a promo for the Game of Thrones season two finale below: