23-year-old Kopech -- who's rehabbin' after a Tommy John surgery last year -- was vacationing with his GF near the Grand Canyon on Wednesday ... when he got on a knee and proposed.

Morgan said yes, calling the stud pitcher afterward, "My Forever."

Kopech and Morgan have been dating for over a year now -- and the two have all the makings of a huge A-list couple.

Kopech, who was previously in a high profile relationship with reality star Brielle Biermann, is the White Sox's top prospect ... and is expected to be a future ace when his right arm heals up.

As for Morgan, she appeared on a couple episodes of Degrassi: The Next Generation back in 2013 ... and she's been blowing up as "Toni" on "Riverdale" the past couple years.

Kopech said of her after the proposal, "I'll never have enough words to tell you how much I love you, but I promise to always you show you."