Ackermann took the Lions to last year's Super Rugby final and has been tipped as a future Springbok coach but joins the exodus of South African expertise to Europe when he replaces Laurie Fisher, who stood down last month, at Gloucester.
The 46-year-old, who won 13 Springboks caps and is coach of South Africa's A team, has been at the Lions since 2013 and led the Johannesburg-based side to 34 wins in 57 Super Rugby games.
He has also been named South African Rugby's Coach of the Year for the last three campaigns.
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"I have come to a crossroads in my career and believe that my coaching journey needs to change in order for me to grow," Ackermann wrote in a letter to the Lions, released to the media on Monday.
"We're all delighted that Johan has agreed to become the next head coach," Gloucester's director of rugby David Humphreys told the club website (www.gloucesterrugby.co.uk).
"He is one of the top coaching talents in world rugby, and it's a significant coup for the club to recruit someone of his calibre."
(Reporting by Mark Gleeson in Cape Town; Editing by John O'Brien)
