The 45-year-old actress said on Thursday's episode of The Ellen DeGeneres Show that DeGeneres' love for wildlife and late primatologist Dian Fossey inspired her to make a grand gesture.
"It's your 60th birthday, and this gift had to be really special. It had to represent who you are and what you really care about," de Rossi said.
De Rossi surprised DeGeneres by announcing she will establish the Ellen DeGeneres Center for the Dian Fossey Gorilla Fund. She also created the Ellen DeGeneres Wildlife Fund to oversee the project and future works.
"It'll be a most incredible center for tourism, education and for further scientific research," the star said.
DeGeneres, who turned 60 on Jan. 26, has been celebrating her birthday on her show with various celebrities over the past two weeks. The television personality was floored by her wife's present.
"It's the best gift," she said. "[Portia] understands me. That is the best gift that anybody could've gotten me."
De Rossi herself celebrated her 45th birthday Wednesday, Jan. 31. DeGeneres marked the occasion by sharing a photo with the Arrested Development star on Instagram, writing, "Happy birthday to my favorite person in the world."