The former Crystal Palace goalkeeper will start against Newcastle United.
He told the club's website: “It obviously happened quite late in the window, but I'm buzzing to be here. It had kind of been spoken about a little, but in football, you never know what's going to happen. When I got the call to drive up here and do the medical on the Wednesday afternoon, I packed the car and set off.
“I did my medical late into Wednesday, it was about midnight by the time I left the training ground, but then I was ready to train on the Thursday.
“It was kind of in the media and I had my own personal situation at Palace, I obviously wanted to play football, but I’d been out with my elbow since March as well. I was training and getting back up to scratch and then this move came around. I’ve known Cuts (Neil Cutler) for a long time and Dezza (Shaun Derry) and I’d heard good things about the manager, so it was as a no brainer.”
Johnstone also said: “It's a great atmosphere for us to play in front of – and obviously not so good for the away team, so hopefully that'll help as well. But the way we play football, we've got a good team, and hopefully we can get some big results at home.
“Playing here for different rivals, I just remember the atmosphere here, playing in the night games, under the lights, the pitch was wet and nice. You've got the flames when we come out and walk out of tunnel, but I’m not sure if I’ve ever won here, so it would be nice to change that at the weekend.
“I can't wait for Sunday and the chance to make my home debut. I’ve got that same nervous adrenaline, energy, and the atmosphere at the Molineux has been brilliant every time I’ve played there, so it'll be good to play here now as a Wolves player. But it's a massive game. Newcastle are a very good team, but so are we. We’ll go out on Sunday to try and get the three points.”