Sources tell TMZ ... Chris asked Hailee to be his wife a couple of weeks ago, as they were en route to the Seattle marathon -- in which Chris was supposed to be running in.

While driving, Chris told Hailee he forgot his bib and made her start heading back to their Airbnb. When she turned down a particular street that was beautifully lit up, we're told he asked Hailee to pull over and step out of the car.

She thought he just wanted to take the wheel, but instead got down and proposed. She said yes ... even though he didn't have a ring at the time.

Frankly, we see that as a sign of true love ... it wasn't about a diamond!

A few days later, Chris and Hailee -- who met working on the same film in 2020 -- told her parents, who offered up her grandmother's diamond for Hailee's engagement ring.

We're told the couple designed the full ring together after that.

As you know, Chris and Paris got engaged pretty fast back in 2018, with her saying "yes" a little less than a year after making things official. They called things off close to a year later, and Paris eventually tied the knot with Carter Reum.