West told friends they were attending a surprise party for Kardashian, who turned 33 on Monday.
He chartered a plane to bring guests to and from the Bay Area and had family and friends hide out in the park’s dugout while he popped the question.
The new bride-to-be had no idea what West was up to.
"Kim thought it was only her and Kanye in the stadium," E! reported. "They all surprised her afterwards."
"Everyone was in tears, it was incredibly romantic. Kim was beaming… she was smiling from ear to ear. Kim and Kanye were so cute together. They were so overjoyed and happy. It was the most perfect thing ever.”
Kardashian was ecstatic following the engagement.
"Last night was truly magical!!! I am the luckiest girl in the world!" she said. "I get to marry my best friend!”
She also posted a photo on Instagram showing her 15-carat diamond ring with a caption that said "YES!!!"