The Rugby Football League has revealed the West Yorkshire club overspent by £55,000 in 2018.
However, Leeds - who are first time offenders - have not suffered a points deduction.
A statement read: "Leeds Rhinos have been fined £20,000, with £10,000 of this suspended, in relation to admitted salary cap breaches during the 2018 season.
"It was established during a routine RFL audit that Leeds had failed to declare payments to three players.
"The total amount of the breach was £55,000, with the club found to have exceeded the finite salary cap at four separate points during the season by sums ranging from £10,500 to £25,500.
"Leeds Rhinos acknowledged these errors and reached an ‘Agreed Decision’ with the RFL in accordance with RFL processes, thereby removing the need for an Independent Tribunal."
Super League clubs were limited to a salary cap of £1.9m in 2018 and £2m this year.
Rhinos chief executive Gary Hetherington responded in a statement: “This was an administrative error on our part for which I take responsibility and we are obviously disappointed to lose our 23-year unblemished salary cap compliance record.
“We had no idea we had exceeded the ‘live’ cap at the time and with hindsight we could have averted the breach by loaning out a player to another club.
“Two of the players, Jack Walker and Luke Briscoe, were promoted from part time to full time status during the season and whilst their increased monthly payments were included in our salary cap return to the RFL, we didn’t lodge their new contracts until the end of the season.
Matt Parcell also had his contract extended at the start of the season and whilst the increased monthly payments were paid the contract wasn’t signed and lodged with the RFL until the end of the season and those combined breaches created the breach we now accept.
“Managing a Super League club salary cap is, and has always been, a very precise operation and we have now revised some of our internal procedures to ensure we don’t make the same mistake again.”
Wigan Warriors were the last club to be found breaching the cap when they were docked two points in January.
It was related to six separate payments totalling £14,700 in 2017 but they won an appeal to have the points reinstated in March.