Kamani and Kane, who founded the group in Manchester, northern England, in 2006, between them sold 4.3% of Boohoo's equity through a placing to institutional investors, reducing their combined holding to 15.8%.
Kamani, the company's executive chairman, sold 35 million shares at 285 pence each, reducing his stake to 13.1%, while Kane, an executive director, sold 15 million shares at the same price, cutting her holding to 2.7%.
The placing was managed by Zeus Capital and Jefferies.
Boohoo has expanded quickly, listing its shares in 2014 and buying the PrettyLittleThing and Nasty Gal brands in 2017. The retailer is tapping into a generation of younger consumers who shop on their mobile phones and share fashion tips through social media.
Boohoo said on Tuesday it had a record performance across the Black Friday period.
Shares in Boohoo, up 84% so far this year, closed on Wednesday at 297.1 pence, giving a market capitalisation of 3.42 billion pounds.
Online retailers like Boohoo are growing at the expense of traditional shopping groups such as the 135-year old Marks & Spencer, whose equity value has slipped to 3.7 billion pounds.
Shares in Boohoo were down 2.5% at 0809 GMT.
(Reporting by James Davey; Editing by Edmund Blair and Jane Merriman)