The stake is understood to have been purchased for $246.6 million and is held by the joint venture WestArt Trust. UniSuper is also the owner of the balance of the centre.
The ACCC last week advised Westfield that in order to secure a stake in the neighbouring Perth shopping centre, Innaloo, it would have to divest its share in Karrinyup.
Settlement will take place on September 13.
Karrinyup is managed by AMP Capital on behalf of the two unit holders and does not hold any equity in the Karrinyup Shopping Centre Trust.