The committee disclosed the determination in a Dec. 7 letter to Las Vegas resident Robert Rose, who had filed a complaint on the matter.
The Associated Press reported this year that Democrat Reid attended three Las Vegas fights between 2003 and 2005 without paying, using credentials provided by the Nevada Athletic Commission, a state agency. Reid was supporting legislation during this period to create a federal agency to oversee boxing, which the commission opposed.
The legislation passed the Senate but died in the House of Representatives.
At the time Reid defended attending the matches, saying it helped him understand boxing regulations, but he acknowledged it did not look right and said he would not do it again.
Senate rules bar senators from accepting gifts worth more than $50 but make an exception for anything paid for by federal, state or local governments. The rules also specify that senators should be wary of accepting any gift where it appears there is a desire to influence official action.
"There is not substantial credible evidence which provides substantial cause for the committee to conclude that a violation within the jurisdiction of the committee has occurred," the committee's chief counsel and staff director, Robert L. Walker, wrote to Rose.
Attendance at the matches "was a matter appropriately left to (Reid's) discretion," Walker wrote. The committee will take no further action, and the matter is dismissed, the letter said.
"What can I tell you? That's Washington," Rose said, speaking about the decision.
Reid spokesman Jon Summers said: "The letter makes it clear that there wasn't a violation, but Sen. Reid realizes that it didn't look right and he's said that he shouldn't have done it and is not going to do it again in the future."
On another ethics issue, Reid has not yet filed revised financial disclosure forms with the secretary of the Senate to account more fully for a Las Vegas land deal that allowed him to collect $1.1 million for property he had not personally owned in three years.
In mid-October, shortly after the AP reported on the 2001 deal, Reid announced plans to revise his financial disclosure forms. Summers said Reid's staff submitted revised forms to the Ethics Committee in November and was waiting for signoff from the committee before filing them.