The 24-year-old Frenchman completed his £2m move from Arsenal on Tuesday.
Southgate said: "Jeremie is an exciting player. I have felt for a long time now that we needed to bring more pace and energy into the side. Jeremie will bring us those two assets in abundance.
"There is no disgrace in being second choice to some of the strikers that were ahead of him in the Arsenal pecking order, but he has never really had an extended first team opportunity.
"He will have the chance with us - and the fact that he is hungry and perhaps has a point to prove can only be a good thing."
Aliadiere has signed a four-year contract and will join up with the team for the start of pre-season.
Southgate said: "I'm looking forward to working with Jeremie and helping him to become the player I know he can be at Middlesbrough."