The pair were first linked in March 2017 when they were photographed walking arm-in-arm in New York City.
They made their first public appearance as a couple in September 2017 when they attended the Gersh pre-Emmys party together.
The Wind River star was previously engaged to Narcos actor Boyd Holbrook. The pair dated for three years before breaking off their engagement in 2014.
The news comes as rumors swirl that her sister Ashley, 33, might also be set to walk down the aisle.
The fashion designer was photographed with a mysterious ring on her wedding finger during a date night with boyfriend Louis Eisner on Monday, July 30.
Her other sister Mary-Kate has been happily married to husband Olivier Sarkozy since 2015.