The Danish dairy giant, which stunned the industry last week by announcing plans for a one billion litre mega-dairy in London, bought out Fonterra's 25% stake in the Anchor joint venture.
The acquisition, for an undisclosed sum, gives Arla full rights to the brand in all categories in the UK, and for Anchor packet butter and aerosol cream in Europe.
Arla and Fonterra also signed an agreement that will see the New Zealand company continuing to supply bulk butter for the Anchor brand and own-label products in the UK.
"Anchor is a very strong, successful brand in the UK and it is a good business in which to invest," said chief executive Peter Lauritzen. "The butter, spreads and margarine market is a key category for Arla and this is a logical step for us."