The youngster, at St Mary and All Saints C of E Primary School in Coley Park, has been sent home to self isolate along with two other children.
The school, based on Wensley Road, is the latest in a recent spate of Covid-19 cases confirmed in schools following their reopening this month.
The news follows the announcement of two cases at Reading schools last week, including another primary school.
A spokesman from Reading Borough Council said: “Individuals or class bubbles self-isolating in this way is a fundamental part of schools re opening in a Covid secure way and a key tactic in helping to contain the spread of the virus in both our schools and in our local communities.
“Public health guidance is in place to ensure it remains safe for schools to open and the council’s Public Health Team and Brighter Futures for Children continue to ensure all relevant guidelines are being followed, which includes children or class bubbles self-isolating when necessary.
“We appreciate the huge efforts being made in schools across Reading in helping to ensure regular handwashing takes place, respiratory hygiene guidelines are followed and the rapid reporting and action on any positive tests among the school community.”