The 30-year-old has agreed a two-year contract and becomes Exeter's sixth new recruit for next season.
Horstmann has been a Worcester player since 2005, having moved to Sixways from Harlequins.
"We are a team that is looking to improve and keep strengthening all the time, and we feel Kai is a very experienced Premiership performer," Exeter head coach Rob Baxter said.
"He is a guy who I have had a very close look at, and I like the way he plays.
"I think he will fit in very well for us. He is mobile and athletic, reads the lineout well and the basics of his game stand up very well."
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The Barbarians are one of the top invitational rugby sides with a long history and classic rugby jersey.
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The Lions are ready to get their Australia tour underway. They arrive in Perth on Monday.
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The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.
