Olivia Culpo and Christian McCaffrey are ready to spend the rest of their lives together ... TMZ Sports has learned they're engaged!!!

The 26-year-old NFL star popped the question to the 30-year-old supermodel while the two were on a recent road trip vacation ... and we're told Culpo said "yes."

Of course, her answer should surprise no one ... 'cause the two have been absolutely inseparable since they began dating back in 2019.

Despite his busy football career -- which featured a trade from the Carolina Panthers to the San Francisco 49ers this past season -- Culpo was spotted with McCaffrey often ... attending plenty of his games and hanging with him and their families in offseasons.

Our sources say McCaffrey had been planning to propose to Culpo for the last couple months -- hitting up a jeweler in New York back earlier this year to get the ring.

No word on a wedding date just yet ... but you can bet it'll be star-studded -- and lavish -- when the two eventually say "I Do."

We reached out to a rep for Olivia .. so far no word back.

Congrats!!!