Ackerson Meadow features wetlands and rolling hills that are home to endangered wildlife, park officials quoted by AP news agency said.
The land, on Yosemite’s western boundary, was bought for the park by conservation group the Trust for Public Land for $2.3m (£1.7m).
Yosemite National Park covers about 1,200 sq miles of mountainous scenery.
It attracts millions of visitors every year and celebrated its 125th anniversary in 2015.
The new addition, which was traditionally used for logging and grazing cattle, will be preserved as habitat for wildlife including the endangered great grey owl, the largest owl in North America, the officials said.
The land had been owned since 2006 by Robin and Nancy Wainwright.
Mr Wainwright said that by selling it to the trust, they had passed up a lucrative offer from a developer which had wanted to build a resort.
He said he had often seen bears strolling through the meadow and owls over fields of wildflowers in the spring.
Mr Wainwright said he hadn’t wanted that experience to be available only to visitors who could afford to stay in a resort.
‘To have that accessible by everyone, to me is just a great thing. It was worth losing a little bit of money for that,’ he added.
Park spokesman Scott Gediman said Yosemite’s boundary had seen some minor changes over the years but the addition of Ackerson Meadow was the largest expansion since 1949.
He said the Trust for Public Land had put up $1.53m for the purchase with the Yosemite Conservancy and anonymous donors making up the rest.