The 29-year-old right-back could scarcely disguise his delight after putting pen to paper on a two-year deal with a further one-year option at Rockliffe this afternoon.
"It's great to be back," says the player who first joined the club as a nine-year-old. "I couldn't be happier really.
"I went to Ayresome Park and watched the games there, then followed them at the Riverside and played there of course, so there's only really been four years out of my footballing life where I haven't been a part of the club."
One of those years was last year when a deal to re-sign with Birmingham City fell through in the summer and he spent the season without a club.
"I was training here last year and I'm grateful for the club for letting me do that. It was a really strange year for me. Perhaps when you are a professional footballer playing at the level I was you kind of take it for granted that if you leave a Premier League club you will get something else. Well, it doesn't work that way!
"A few things could have happened which didn't, I was close to a few clubs, without ever getting across the line. But it did me no harm to go through that. I probably worked as hard as I ever had last year and hopefully the fans will see that.
"You see the list of out-of-contract players now and it's never been longer. I'll never take anything for granted again and I don't think any player can."
Parnaby remained positive throughout his time away from the game, adding: "I suppose if I had been down in the dumps I may never have got anything.
"Being out of the game has made me realise how lucky I am to be involved. I have had a mental kick up the backside, I've never been more positive and ready to go that extra mile for Middlesbrough."
Parnaby says he hasn't been given any guarantees by Tony Mowbray, emphasising: "I've always had to fight for a place, wherever I have been, and this is no different.
"Obviously I want to make the place my own, ideally in a Middlesbrough side managed by a Middlesbrough man, playing in the Premier League. That's where the fans want to be and that's where we all want to be."
Durham-born Parnaby began his career with Boro and was earmarked for a big future from an early stage, captaining his country at various levels. He made his full debut for Boro in the League Cup win over Mansfield Town in September 2000 and made his full Premiership debut in the 2-2 draw against Leeds United at the Riverside in October 2002.
He moved to Birmingham City in 2007, signing a three-year deal that was extended by a further year.
Parnaby played a vital role in securing Boro's first ever major trophy win, pressurising Arsenal's Jose Antonio Reyes to score the own goal that put the side 2-1 up in the Carling Cup semi-final second leg at the Riverside.
He was also part of the team that played in the 2006 UEFA Cup final against Sevilla at Eindhoven.As well as at full-back he can also operate in the centre of defence or in midfield.
Parnaby's signature follows the capture of midfielder Grant Leadbitter from Ipswich Town earlier in the summer.