NHS data from the end of December reveals 589 youngsters had been on the waiting list for 52 weeks or more to get an appointment with Child and Adolescent Mental Health Services (CAMHS).
A total of 272 children and young people who were seen for the first time in the last three months of 2019 had waited over a year for their appointment.
Opposition MSPs said the statistics showed the Scottish Government was “failing a generation of young people”.
By the end of December, there were 10,820 children and young people waiting for CAMHS help – up from a total of 9,337 the previous year.
According to the report, the number waiting for treatment has gone from a low of 7,620 in the last three months of 2017 to a high of 10,820 at the end of the most recent quarter.
The number who started receiving help meanwhile fell.
'Around the clock'
Liberal Democrat health spokesman Alex Cole-Hamilton said: “At the same time as these unbearable waits were being recorded, SNP ministers at Parliament were voting to deny that there is a mental health crisis in Scotland.
“These statistics show how wrong that was.
“Staff are working around the clock, but they aren’t getting anywhere close to the support and resources they need.”
Over October to December last year, 3,884 children and young people started treatment with CAMHS, a fall of 14.1 per cent from the total of 4,523 that had been recorded in the same period in 2018.
The Scottish Government has set the target for 90 per cent of patients being seen within 18 weeks, but last year there were 5,027 children and young people who waited longer than this to start receiving help.
In the final three months of the year, the target was achieved for just two thirds (66.4 per cent) of youngsters – down from the 72.8 per cent who were seen within this time in the last quarter of 2018.
The most recent figures showed while some health boards exceeded the 90 per cent target – NHS Borders, NHS Orkney and NHS Western Isles – others did not.
Fewer than three out of five CAMHS patients were seen within 18 weeks at three health boards.
NHS Tayside saw just 57.4 per cent of youngsters within this target time, while NHS Forth Valley and NHS Lanarkshire achieved 57.7 per cent and 57.8 per cent respectively.
For NHS Lothian, more than a quarter (27.2 per cent) of patients who started treatment in period October to December had waited more than a year.
Labour MSP Mary Fee said: “These new figures show what we have known now for some time – the SNP is failing Scotland’s young people.
“At a time when youth suicides have been increasing, these figures should shame SNP ministers into action.”
Children’s campaigners at Barnardo’s Scotland stressed the CAMHS statistics were “just a small part of the bigger picture of children and young people’s mental health and well-being”.