They have bought Cyfronydd Hall, near Welshpool, which was on offer at Â£2.5m.
"Ffion and I are looking forward to having a home in Wales for the first time and to living in such a beautiful part of the country," Mr Hague told the local County Times newspaper.
Their new home has 10 bedrooms, 10 bathrooms and 12.7 acres (5ha) of land.
Mr Hague met his wife while he was Welsh Secretary in the 1990s and she worked in his private office.
"Ffion is very excited that we will be living just over the hill from Meifod where the National Eisteddfod will be held later this year, we are both looking forward to that," he said.
Mr Hague added that his new home was "an ideal place to write books".
Both he and his wife are published authors.
Montgomeryshire Tory MP Glyn Davies said he was "really pleased" the Hagues had decided to set up their family home in the area.
"Ffion's father was a Montgomeryshire man so she has family roots here, and William is hugely popular throughout Wales," he said.
Mr Davies added that Cyfronydd Hall, built in 1865, was a well-known local landmark which during his childhood had been a girls' school.
Former Tory leader William Hague and his wife Ffion will move to Powys when he retires from frontline politics at the general election.