"Learnt" is common in the UK, but unusual in the US. (The spelling checker in FIrefox here just put a wavy red line under it.) Even in the UK, some people distinguish the past tense, for which they use "learnt", and the past participle, for which they use "learned". Similar considerations apply to some other verbs too; "leant"/"leaned" and "lit"/"lighted" come to my mind.

Both are correct, 'learned' being more common in American English, and 'learnt' being more common in British English.