Harrison, who was scheduled to enter free agency after this season, is guaranteed $22 million between now and 2006 and received a $6 million signing bonus. He is making $5.56 million this season.
Earlier this year, the Colts locked up quarterback Peyton Manning with a seven-year, $98 million deal that included a $34 million signing bonus.
Harrison, 32, is considered one of the best receivers in the league and is the focal point of the Colts' powerful offense. He draws consistent double coverage and allows Reggie Wayne and Brandon Stokley to excel against single coverage.
The structure of Harrison's contract will allow enough flexibility for the Colts to try to find a way to keep running back Edgerrin James. There is a $7 million roster bonus that can be converted into a signing bonus in order to lower Harrison's cap number in 2005 to around only $3.2 million.
There also is a $10 million flexible roster bonus that can be converted into signing bonus to help the salary cap in 2006.
The Harrison deal is considered the best ever given to a wide receiver. Randy Moss had the previous best deal for a player at the position, an eight-year, $75 million contract that included $18 million in guarantees.
The surprising part about Harrison's contract is that he received it at age 32. Though many top receivers have some of their best seasons in their 30s, a lot of teams don't like giving big-money deals to older receivers.
The Colts had no problem doing this because Harrison has been such a valuable part of their offense along with the fact he is one of the best-conditioned athletes on the team.
Negotiations concluded Tuesday night and the deal was sent into the league office Wednesday morning for approval. Once it was approved, it went into the league's salary cap system.
Harrison's agent, Tom Condon, was unavailable for comment.
Condon also negotiated Manning's deal, which was the most given to any quarterback.
Along with James, Stokley will be among the Colts' next targets for re-signing. He's a free agent after this season.
Although the contract technically has $23 million of guarantees, the deal counts only as $22 million because the Colts lowered $1 million of his remaining contract to fit it into this year's cap.
Senior writer John Clayton covers the NFL for ESPN.com.