26-year-old Lindsay -- the 1st undrafted offensive player to make the Pro Bowl as a rookie back in 2018 -- popped the question to longtime GF Morgan Stanton ... and she said yes!!

Lindsay and Stanton were having dinner at a fancy restaurant ... when the NFL star dropped down on 1 knee and pulled out the big rock.

The couple has been together for a long time. They both attended the University of Colorado. Morgan was on the soccer squad ... and Lindsay was a star on the football team.

Stanton also gave birth to the couple’s first child last year.

FYI, Lindsay is set to be a free agent after a red-hot start to his career ... registering back-to-back 1,000-yard rushing seasons. It’s unclear if he’ll return to the Broncos.

Stanton announced the engagement on social media, saying ... “You have exceeded every expectation I have ever had in terms of friend, partner and parent. So lucky I get to love you forever.”