Brill initially joined the Highlanders on a temporary basis from Luton in July until January.
Caley Thistle negotiated with the English club to release the 27-year-old from his contract so that he could commit himself to the Scottish Premiership side.
Chairman Kenny Cameron said: "Discussions with Dean and Luton have been ongoing but they have now been brought to a highly satisfactory conclusion.
"Luton have cancelled the remainder of his two-year contract, which was due to expire next summer.
"That has cleared the way for us to reach agreement with Dean and I am extremely pleased that he has signed a two-and-a-half year contract with us.
"Dean has been very good to deal with and our supporters will be delighted that he has put pen to paper."
Brill added: "Coming to Inverness has turned out to be a great move for me.
"I enjoy living here and there is a great dressing room solidarity.
"I was very pleased to agree a long-term deal with the chairman and commit my future to Caley Thistle."