The 27-year-old, who had been on loan at the Scottish club from Luton, has kept six clean sheets during an impressive start to the season.
Inverness chairman Kenny Cameron told the club's official website: "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 three-and-a-half-year contract with us."
Brill had been with Luton since 1995.