The 26-year-old, who has joined the club from Cardiff, has signed a three-year contract that will keep him at the club until 2012.
Johnson is the third centre-half the club has signed this summer.
“Competition is good,” he told the club’s official website. “We had it at Cardiff when I first signed and I had to battle my way into the side.
“I am not frightened to do that. I am not expecting to walk into the side. I will battle it out with the other boys and I am sure they are thinking the same.”
He added: “The fee in a way does add a bit of pressure but I will just do what I will do.
“It is a massive fee but I take the pressure in my stride and just crack on. Hopefully I will do as well as I did at Cardiff and do the same for Birmingham.”