A primary school teacher has been charged with indecently assaulting three nine-year-old girls at a school in Blacktown in western Sydney.

The alleged assaults happened at the school last year and police became aware of the claims in November.

Detectives from the Child Abuse Squad arrested the 23-year-old man at Valley Heights at 1pm on Tuesday.

He has been charged with three counts of aggravated indecent assault of a victim under authority of the offender.

He has been granted conditional bail to appear before Blacktown Local Court in March.

Police said the man was a teacher at the school at the time of the alleged assaults, but no longer works there.

-AAP