Lamar made his engagement announcement two days after the release of the music video of his recent single "King Kunta," which is featured on his new album, To Pimp A Butterfly.
He and Alford have been together for about 10 years.
In an interview with The New York Times last year, when asked who in his life has the power to call him out, he said it was Alford.
"She's been here since day 1," he said.
"I wouldn't even call her my girl," Lamar had told Billboard in an interview published in January. "That's my best friend. I don't even like the term that society has put in the world as far as being a companion—she's somebody I can tell my fears to."