The British super-featherweight champion was one of three Smith brothers set to defend their titles on a card in Liverpool on December 7.
Paul Smith is the British super-middleweight champion while Liam recently won the light-middleweight equivalent against Erick Ochieng.
The 28-year-old Stephen, however, has penned a three-fight deal with Hearn after walking away from his W Promotions contract following the company’s voluntary administration.
“I’m really looking forward to pushing on with my career with Eddie,” said Smith. “It’s the best move for me, Eddie’s got some great plans and I’m really excited by the future.”
He is set to debut for his new backers on the undercard of Carl Froch v George Groves on November 23, defending his Lonsdale title against Gary Sykes.
“My fight with Gary has the makings of a really great clash,” said Smith. “Our styles will gel well and the fans are in for a cracker on a great bill, and of course the British title is really important to me and to all of us in the family.
"I won’t be looking beyond Gary but there are some really interesting fights down the line for me as I look to move onto the world stage. I hope to be able to put on a great display against Gary in what will be a really tough fight, then push on to some big battles next year.”
The champion made clear his intentions to switch promoters prior to choosing Hearn.
“I’ve been with W Promotions since I turned pro and during that time have held the British featherweight and super-featherweight titles,” ‘Swifty’ Smith said last week.
”Now though the time has come to pursue my career away from W Promotions.
“As such I’m today announcing the termination of my contract with W Promotions. I would like to thank everyone at W Promotions for the support and guidance they’ve given me."
Warren responded by denying that Smith was free to leave and promised legal action.
“Mr. Warren will defend his rights as both the Manager and Promoter of Stephen Smith and a complaint has already been lodged at the British Boxing Board of Control and a letter sent to his solicitors,” a rebuttal statement read.
“Other parties, who we are aware have induced Mr. Smith to breach his contract and in doing so have tortuously interfered with Mr Warren’s rights, will also be the subject of action through the BBBofC and the Courts.”
Another Smith brother, Callum, is already under the Eddie Hearn umbrella and is the reigning English super-middleweight champion.