Terms of the deal were not available, although it is believed to be for a late-round, conditional draft choice in 2007.
The report, which stemmed from an anonymous source, said the two sides completed the deal late Thursday, forcing Harrington to forego a scheduled charity event next week and join the Dolphins for the team's opening day of organized activity.
The deal concludes what has been weeks of speculation and frustration for Detroit, Miami and Harrington. In early April, Harrington announced publicly that he would play in Miami next season. However, that was before a deal was completed.
During the draft, the Lions attempted to deal Harrington to Miami, Cleveland and Kansas City, but could not obtain what they felt was just value. Harrington was the No. 3 overall pick in 2002.
Harrington could start for the Dolphins if former Viking Daunte Culpepper is not yet fully recovered from a knee injury suffered in 2005.
The Lions will host the Dolphins during the annual Thanksgiving Day game on November 23rd.