Young people are far more likely to be arrested if they are black, Asian or of minority ethnic status (BAME).
While only 12 per cent of the county’s population is BAME, a quarter of all youth arrests are of BAME children.
The figures emerged as part of the Lammy Review, an independent study of the treatment of, and outcomes for, BAME individuals in the criminal justice system.
And Frances Crook, chief executive of the Howard League for Penal Reform, said it was a time for a wider debate.
He said: “The disproportionate number of BAME children being brought into the system is of great concern.
“It raises serious questions about decision-making throughout the criminal justice journey – from the police’s decision to arrest, to the remand and sentencing decisions of the youth courts.
“The Lammy Review has called on police forces and other criminal justice agencies to either explain disparities or reform.
“Our analysis of child arrests data is intended to assist this discussion.”
Herts Police had the fourth-highest proportion of BAME youths arrested out of 43 police forces in England, not including Thames Valley which did not provide figures.
A spokesman for Herts Police said: “Herts Constabulary is focused on forming a better understanding around any disproportionality in relation to the arrest of children from a BAME background across the county.
“This review led by MP David Lammy has justly shined a spotlight on the issue of disproportionality within forces across the country and we recognise that there is much work that needs to be done in order to tackle this.
“In August, we launched a recruitment drive to focus on enrolling more officers from a BAME background in order to greater represent all parts of society.
“Ultimately, by recruiting more officers from a BAME background we seek to gain a better understanding of all the communities that we serve in Hertfordshire.
“However, this is a complicated issue and not one that is isolated to the police alone – it is the responsibility of the entire criminal justice system and society as a whole to address disproportionality.”