Set in Auckland, Harry follows Detective Harry Anglesea (Oscar Kightley, Sione's Wedding), a single father struggling with his personal demons as he investigates a major murder case.
Neill, last seen on television screens in the United States mystery drama series Alcatraz, will play Detective Jim "Stocks" Stockton, Harry's chief and mentor.
Kightley said in a statement he was "pretty excited" about the series and having real-life friend Neill co-star "lifts that excitement to another stratosphere".
"It is a massive honour for us that he's chosen this project."
Neill said he was "very much looking forward to coming home to work on what I think should be a ground-breaking series from New Zealand".
Producer Steven O'Meagher said draft scripts for Harry, co-written by Kightley, piqued the interest of Neill, who had never acted in a New Zealand drama series. Negotiations took place over a month, while the actor worked on productions in the United Kingdom and US.
"Sam Neill is one of our greatest-ever actors and his agreeing to play Detective Jim 'Stocks' Stockton is a huge endorsement on the quality of the series," O'Meagher said.
Harry is being produced by Desert Road for TV3 with funding from New Zealand On Air's Platinum Fund, which is reserved for programmes of historical or cultural importance with significant relevance to New Zealanders.
