The monument, at White Brae, Ligoneil, commemorates three fusiliers from Scotland who were killed in March 1971.
John MacVicar, from the local Royal British Legion branch, said paint has been poured over the memorial and on wreaths that had been left by mourners.
He said vandals also tried to break the granite in the attack on Tuesday night.
The memorial was erected three years ago at the site where the bodies of teenage brothers John and Joseph McCaig, and their 23-year-old colleague Dougald McCaughey, were found more than three decades before.
Mr MacVicar, who helped raise funds to build the monument, is the secretary of Oldpark/Cavehill Royal British Legion branch.
He said he believed it was a "sectarian hate attack".
"This is now the fifth time the memorial has been attacked since it was unveiled in 2009.  The damage caused the last time cost in excess of Â£1500 to repair," he said.
Mr MacVicar claimed: "There is absolutely no doubt that this is a further attempt by republicans to remove from history the fact that the IRA murdered three young boys, two brothers aged 17 and 18 and the third aged 23 who were unarmed and out for an off duty afternoon in Belfast in 1971 simply because they were in the British Army."
A spokesman for the Police Service of Northern Ireland (PSNI) said they had not yet received a report of the attack.
Last July, sectarian graffiti was scrawled on the memorial stone. Corner posts, and the surrounding rope were also broken.

Labour's Clive Lewis, who is standing in Norwich South, made the comment in an interview with the New Statesman.
He had been asked if he was taking victory in the marginal seat for granted.
He said he was "sincerely sorry" if anyone had been offended by the comment.
Mr Lewis, who worked for BBC Look East as a reporter, had told the magazine: "I mean, in the multiverse there's still three universes in a hundred where there's a Green MP in Norwich, so anything could happen.
"I could be caught with my pants down behind a goat with Ed Miliband at the other end - well, hopefully that won't happen."
In 2010, Labour's Charles Clarke lost the seat to the Liberal Democrats' Simon Wright who had a majority of 310 votes.
Regarding his New Statesman interview, Mr Lewis said: "On occasion we all have the ability to get carried away with language and colourful metaphors.
"If anyone was genuinely offended then I'm sincerely sorry for that."
Mr Lewis is standing against independent Cengiz Ceker, UKIP's Stephen Emmens, Lesley Grahame from the Green Party, David Peel of Class War, Conservative Lisa Townsend and Liberal Democrat Simon Wright.
A memorial to three soldiers murdered by the IRA on the outskirts of Belfast has been vandalised for a fifth time, a local fundraiser has said.
[NEXT_CONCEPT]
An election candidate has apologised for joking that he "could be caught with my pants down behind a goat with Ed Miliband at the other end".