The versatile Whitehead, who played for Sunderland and Stoke City in the Premier League, is Town boss Chris Powell’s first signing of the summer.
Whitehead joins from Middlesbrough, who he represented in the Championship play-off final last month.
He links up with former Boro team-mate Jacob Butterfield at Huddersfield.
Powell said: “Bringing Dean Whitehead to the club was among my top priorities during this transfer window, so I’m delighted to welcome him to Huddersfield Town today.
“His arrival is testament to our policy of staying patient to ensure we recruit the right players for this club.
“The Championship is a tough and relentless division and the addition of Dean will mean we are better equipped to deal with the challenges that brings.
“Dean has proven ability and quality that will improve us. He has a vast wealth of experience at the top level of English football. He made a big contribution to a team that was excellent last year and was defensively strong.
“He’s actually played more games than our other senior midfielders – Jacob Butterfield, Conor Coady and Jonathan Hogg – have combined.
“As well as that ability, I have often talked about how important organisation and game management is and I believe Dean will help to us that in the vital midfield area of the pitch for us, whether that is helping to lead us through difficult periods in a game or seeing out a game. He will complement the players we already have and add to the competition and depth of the group and use his experience and know how to help our younger players too.”