Rocket Internet said it would buy back up to 10% of its share capital in an offer starting on Dec. 10 and ending on Dec 18, with United Internet committing to accept an offer for its 7.4% stake.
Berlin-based Rocket will offer 21.50 euros per share, valuing its buyback at up to 324 million euros (£278.34 million). Its shares were trading up 4.2% at 22.66 euros at 0945 GMT. United Internet's shares were down 0.4% at 30.55 euros.
Rocket Internet said in a statement that its founder and Chief Executive Oliver Samwer would himself buy any part of United Internet's stake not purchased by Rocket over the next six months if more shares are tendered than the 10%.
In September, Samwer said Rocket Internet had no immediate plans to go private, dismissing speculation that he was planning a delisting in order to be able to make more autonomous investment decisions.
Samwer and his family currently own almost 44% of Rocket and the latest buyback would push their holding towards 50%.
A spokeswoman for Rocket declined to comment on Monday on the reason for the buyback.
United Internet is a telecoms and internet company based in Montabaur outside Frankfurt.
"The stake in Rocket Internet no longer has any great strategic value for us," a spokeswoman for United said.
Rocket has faced repeated questions about what it plans to do with its cash pile, which stood at 2.6 billion euros as of Nov. 8, now that it has listed most of the major start-ups it launched.
Separately, United Internet said it would buy back up to 9 million shares at 29.65 euros each, with Rocket Internet pledging to accept the offer for 8,135,804 shares it holds in the company, about 4% of the current share capital.
United's share buyback will be worth 267 million euros.
Samwer and Ralph Dommermuth, the billionaire CEO of United Internet, have long had intertwined fortunes.
United first took a stake in Rocket in 2014 just weeks before Rocket went public, but had been working with Samwer and his brothers through a venture capital fund called Global Founders Capital since 2007.
Rocket's stake in United was only declared in November when it crossed the 5% threshold.
German business monthly Manager Magazin highlighted the crossholdings last month after Rocket Internet declared a 5.46% stake in United Internet and also invested in Tele Columbus, a telecoms company in which Dommermuth is the biggest shareholder.
Dommermuth holds a 29.7% stake in Tele Columbus and Rocket Internet took a 12.29% stake in October and said it would seek to add more over the next 12 months.
(Reporting by Emma Thomasson; Editing by Tassilo Hummel and Susan Fenton)