Hamilton announced the news on Instagram where she posted a picture with the filmmaker sporting a sparkling, Monique Pean ring as Chazelle plants a kiss on her cheek.
The director, 32, had referred to Hamilton as "the love of my life", while accepting the award for Best Director at the Critics Choice Awards in December last year.
"This is a film about love, I think, more than anything. So I have to thank the love of my life, Olivia Hamilton," he said of "La La Land" at the time.
Chazelle was previously married to Jasmine McGlade, an executive producer on La La Land. They parted ways in 2014.