The 21-year-old Maynard came through the ranks at Glamorgan, playing in 93 first-team games and making two centuries in one-day games.
Surrey cricket manager Chris Adams said the opportunity to sign Maynard, though unplanned, was too good to pass up.
He said: “Once we received permission from Glamorgan to speak to Tom, it became clear that Surrey was his preferred destination.
“He has been attracted here by a number of factors including the wicket at the Kia Oval and the fantastic history of the club but most of all he had a sense of wanting to be at Surrey through what will be a very exciting time here.
“We were painfully on the receiving end of what he is capable of last season and although this was not a signing we had planned for, it was an opportunity that was too good not to take and I am delighted that Tom is adding further depth to what is looking like a very strong squad.”
Maynard will add to an already youthful look to the Surrey batting line-up, and one he believes can compete for years to come.
He said: “I’m absolutely delighted to be joining Surrey County Cricket Club at such an exciting time and am looking forward to playing with some highly regarded young talent as well as soaking up as much as I can from the richly experienced and successful coaching staff.”
In a statement Glamorgan said: “(We)would like to thank Tom Maynard for his contribution to the Club and wish him well for the future.
“This announcement now enables both Tom and Glamorgan Cricket to focus on the coming season.”