Radrizzani, 42, who co-founded sports media rights company MP & Silva in 2004 and has strong connections in China, entered into talks with Cellino last summer.
Speculation over the club’s future had been mounting since Cellino assumed 100 per cent control at Elland Road in September when he bought the minority stake still held by previous owner Gulf Finance House.
“I am making a long-term commitment to Leeds United and will work to bring stability through ongoing investment,” Radrizzani said.
“I aim to bring sustainable growth. I won’t do anything that will put the club’s future at risk.
“Through working in the sports industry for many years, I have developed a great passion for the English game and I am honoured to have become joint owner of one of the country’s biggest clubs.”
Cellino’s company Eleonora Sport Ltd had bought 75 per cent of Leeds’ shares from GFH in April 2014 for a reported sum of around £30million.
“I am excited by the challenge ahead and I will work alongside Massimo and everybody at the club to make Leeds United as successful as possible,” Radrizzani said.
“I am fully aware of the great heritage and traditions of Leeds United and I will endeavour to be a fitting custodian on behalf of the many thousands of Leeds supporters, who are the lifeblood of the club.”
MP & Silva owns the exclusive distribution rights for the Premier League in south-east Asia until 2019.
It sold a 65 per cent stake in the company to Chinese investment group Everbright Securities and internet entertainment firm Baofeng in May when it was reported to be valued at £685million.
New owner on Garry Monk
“I am very impressed with the job Garry Monk has done this season and I will do all I can to support him and the team moving forward,” Radrizzani added.
“The responsibility is on myself, Massimo and the executive management at the club to work together over the coming months to provide the best platform and environment for Garry and the team to achieve success.”
Cellino is due to start an 18-month ban from all football activities on February 1 for breaching agent rules over the sale of Ross McCormack to Fulham in 2014.
The Italian and Leeds were both fined £250,000 by the Football Association, while Cellino has already indicated he will appeal against the ruling.
If unsuccessful, Cellino’s suspension means he will not be able to return to any position at Leeds until September 1, 2018.