Kaitlyn Bristowe, a previous Bachelorette who's co-hosting the current season of the show, also commented, writing that Harrison is "truly irreplaceable as a host."
"Thank you for all the years you put into this show and making it what it is," Bristowe continued. "No one does it quite like Mr. Chris Harrison."
Former Bachelor lead Arie Luyendyk Jr. wrote, "Love you brotha. On to bigger and better things."
Bachelorette season 8 star Emily Maynard commented, "I hope you were on a yacht in the South of France when you wrote this. You deserve it! Can't wait to see what’s next and sending you a big hug ❤️❤️❤️."
Chris Bukowski, a former contestant, commented, "Thanks for all the memories Chris. For me you were always a familiar and comforting presence over my last 10 years of being a part of this franchise. Thanks for being a friend and I'm looking forward to seeing what you do next."
Catherine Giudici, who met her husband, Sean Lowe, on The Bachelor, wrote, "Love you @chrisbharrison. You have a lot to be proud about. Can't wait to support you in your next endeavors."
"It will never be the same," Lowe commented. "Love you buddy."
Ben Zorn, a Bachelorette alum, agreed, writing, "The show won't be the same without you! Can't wait to see what is next for you. Good luck my friend!"
Ashley Iaconetti, whose love story with husband Jared Haibon played out on Bachelor in Paradise, expressed the same sentiment, writing, "It will never be the same."
"Truly the end of an era," she continued. "Thank you for all the years of hard work and love you put into the show. I have a lump in my throat writing this. Can't wait to see your next move. I love you."
Bachelor Winter Games alum Lesley Murphy commented as well, writing, "Love you, Chris."
"Won't be the same without you," she wrote. "Cheers to all you've accomplished thus far and excited to see what's next. Wherever and whoever it's with, they'll be lucky to have you!"
Former contestant Eric Bigger chimed in too, writing, "Love you, Chris! Stay true to you."
Kevin Wendt, who met his pregnant fiancée, Astrid Loch, on Bachelor in Paradise, wrote that Harrison's "the OG icon," adding, "We love you, brother."
Jason Mesnick, a former Bachelor, wrote, "Congrats on the run buddy."
"The general public doesn't have any idea how much you helped us cast members deal with the behind the scenes of the TV world," he added. "Looking forward to seeing you soon."
Former Bachelorette Desiree Siegfried wrote, "Wishing you the best in your next chapter."
Kelley Flanagan, who dated Bachelor Peter Weber after appearing on his season, wrote, "You will be missed Chris!"
Former Bachelorette Becca Kufrin wrote that she's wishing Harrison "the best in your next chapters" and sending "so much love."
Longtime Bachelor Nation star Michelle Money wrote, "You are incredible Harrison. What a loss for the show. Many people won't be watching anymore. Myself included. One thing I know is that you will rise above all of this! Can't wait to see what is next for you! I am proud to call you my friend."
Bachelor in Paradise alum Adam Gottschalk wrote, "Can't wait to tee it up with you soon 🏌🏼♂️ Looking forward to what’s next for you 👌🏼."
J.P. Rosenbaum, who won Ashley Hebert's heart in season 7 of The Bachelorette, commented, "❤️ You."
"Love you Chris," former Bachelor Colton Underwood wrote. "Thank you for your friendship."
Underwood also posted on his Instagram Story, calling Harrison a "grade A human being."
"Thank you for being a friend, a stand up guy and a hell of a host," Underwood added. "Can't wait to see what you do next!"
In a statement to ET on behalf of The Bachelor franchise, Warner Horizon and ABC Entertainment said, "Chris Harrison is stepping aside as host of The Bachelor franchise. We are thankful for his many contributions over the past 20 years and wish him all the best on his new journey."
Along with Bristowe, Tayshia Adams is currently hosting Katie Thurston's season of The Bachelorette in Harrison's absence. Earlier this month, a source told ET that David Spade and other guest hosts will lead the upcoming season of Bachelor in Paradise.
As for Michelle Young's season of The Bachelorette, ET has learned that there are no decisions yet on who will host. In the meantime, ET has learned, producers are still focused on casting celebrity guest hosts for Bachelor in Paradise, as it's set to begin filming later this month.