Challenger will book a pre-tax profit of 40 million Australian dollars from the sale in the 2016 financial year, it said in a statement.
This is the first time that Challenger has sold down an equity stake in one of its 14 boutique funds, Chief Executive Officer Brian Benari told Reuters in a telephone interview, in a transaction he calls "unusual".
"We're looking from a longer-term perspective," Benari said. "We see this as building a relationship with Janus that could give rise to opportunities around product and distribution."
Janus is spending at least $85 million to buy a majority interest in Kapstream, which managed $6.6 billion in assets as of March 31.
Shares in the Australian company, which manages A$60.4 billion in assets, rose 1.5 percent to A$6.89 in early trading on Thursday, outperforming the benchmark index that was up 0.8 percent.