The 165-hectare (408 acres) Dunsbury Farm borders the wildlife-rich chalk downland of Compton.
The trust intends to develop the farm habitat to support species including the Glanville fritillary.
Manager Tony Tutton said the Â£2.7m purchase was a "crucial piece of the coastal jigsaw" for the trust.
The trust said it was its largest coastal acquisition in England since 1993.
The Isle of Wight is home to the UK's main population of the Glanville fritillary, and Compton Bay is a traditional stronghold.
The butterfly relies on crumbling cliffs, and the downs behind the coast provide additional breeding habitat.
The National Trust said it would work in partnership with Butterfly Conservation to create the right conditions to safeguard its habitat.
Source: Butterfly Conservation
"Given time and lots of hard work the farm will also become a vital place where we can combine people's enjoyment of butterflies and farmland birds with the stunning views along the chalk cliffs towards the Needles."
Funding for buying the farm came mainly from the National Trust's Neptune Coastal Campaign appeal.
The habitat of a rare butterfly on the Isle of Wight is to be protected following a major acquisition of farmland by the National Trust.