Team president Phil Jackson announced on Thursday that the former NBA guard has been named the Knicks' 28th head coach.
Hornacek agreed to a three-year deal worth $15million, according to reports.
"Jeff has a tremendous basketball acumen and possesses strong leadership skills," Jackson said in a release.
"During his career as both a player and coach, he has demonstrated the ability to elevate the game."
Hornacek was fired by the Phoenix Suns on February 1 after going 101-112 in two and a half seasons.
The 53-year-old did not make the play-offs in Phoenix, even with a 48-34 record in his first season.
Knicks fans know all about that plight, as their team has averaged 29 wins over the past three seasons under Mike Woodson, Derek Fisher and interim coach Kurt Rambis.
Stability on the bench has long been an issue at Madison Square Garden, as no Knicks coach since Jeff Van Gundy (1996-2001) has managed to stay employed for more than three and a half seasons.
Hornacek will be the next man to try and buck that trend.
"I am extremely excited and honoured to be the next coach of such an historic franchise,” Hornacek said.
"I look forward to working with Phil — a coach and teacher of the game I have admired for many years — and collaborating with him and our staff to take this team to the level that Knicks fans expect."