Berahino scored twice as England beat Moldova 3-0 on Tuesday evening, and Southgate was quick to praise the West Bromwich Albion forward, who scored eight times in eight matches in European Championship qualifying Group 1.
"I think he's reaping the rewards for his hard work, as I know he came back early to work with his own fitness coach during the summer," Southgate told the Football Association's website.
"We've been talking about what elite sportsmen act like, with their behaviours and their habits and the rewards that you get if your approach is as good as his has been this summer.
"He gave us a different threat and new options tonight, he plays in that position with his club, but with Harry [Kane] in the side as well, that's a real goal threat and it was exciting to see."
England will learn who they meet in the qualifying playoffs when the draw for the next stage takes place on Friday.