A key figure at Red Bull for almost two decades, Newey's influence has been integral to their success, helping deliver seven drivers' titles and six constructors' championships over two dominant eras.
In a statement released by Red Bull's official website, Horner outlined the impact Newey's departure will have on the team, and paid tribute to who he described as a 'true legend'.
"All of our greatest moments from the past 20 years have come with Adrian’s hand on the technical tiller," he reflected.
"His exceptional ability to conceptualise beyond F1 and bring wider inspiration to bear on the design of grand prix cars, his remarkable talent for embracing change and finding the most rewarding areas of the rules to focus on, and his relentless will to win have helped Red Bull Racing to become a greater force than I think even the late Dietrich Mateschitz might have imagined.
"More than that, the past 19 years with Adrian have been enormous fun.
"For me, when Adrian joined Red Bull, he was already a superstar designer. Two decades and 13 championships later, he leaves as a true legend.
"He is also my friend and someone I will be eternally grateful to for everything he brought to our partnership.
"The legacy he leaves behind will echo through the halls of Milton Keynes and RB17 track car will be a fitting testament and legacy to his time with us.”