"We are thrilled to share the announcement of our engagement! In our favorite place, with close friends and Rio, Dylan popped the question in front of Mount Sopris," Sarah captioned a slideshow of photos of the marriage proposal on the rocks by the water.
Sarah, wearing a plaid shirt and matching pink hat, and Dylan were apparently joined by their dog Rio and close friends in the Colorado mountains.
"To everyone who thinks life has to go in one particular order, or by a specific time... IT DOESN'T. I'd wait a lifetime all over again -- through the heartbreaks, years of self-work and countless Rose Ceremonies -- to end up with this person."
Sarah continued in her post, "We make each other better, we have so much fun and we're going to become parents together, so we decided to do the damn thing!"
"5.15.21 My forever paradise, favorite adventure bud and BEST friend, @dylan.h.brown," she concluded.
For Dylan's part, he posted similar photos Sunday on his own Instagram account.
"Yesterday, I did a thing... And it felt damn good, I must say!! This woman is strong, creative, beautiful (of course), and incredibly patient, to name just a few attributes," Dylan wrote.
"Four years ago I told her I believed we could create an incredible life together and well, I must say I was right! Our life is pretty f-cking special. For those of you who are wondering, she said yes! Thanks for the pics @farm.use // #loveyoyrlife #coloradoliving."
One month later, Sarah and Dylan went Instagram official when she shared a picture of the couple and gushed, "Happily not returning to Bachelor in Paradise this summer." She added the hashtags "Paradise Found" and "adventurebuds" to her post at the time.
Sarah also previously appeared on Season 1 of Bachelor in Paradise in 2014, when she dated Robert Graham, followed by Season 3 of BIP when she found herself caught in a love triangle with Daniel Maguire and Christian Bishop.
Sarah chose to date Daniel, a wise-cracking goofball, instead of Christian -- a decision many fans criticized because Christian had treated Sarah wonderfully on a date in Mexico and seemed to have honorable dating intentions.
Although Daniel did appear to develop genuine feelings for Sarah, he admitted to cameras he just needed a rose in order to stay on the ABC reality dating show longer.