One man was charged with the robbery, one of the biggest in UK history, in a Belfast court on Friday. Another was charged on Monday with attempting to pervert the course of justice by giving false statements in relation to the theft.
"A 22-year-old man was arrested ... this morning in connection with the Northern Bank robbery investigation," a police spokesman said.
Senior police officers and politicians have blamed the December robbery on Irish Republican Army (IRA) guerrillas, who waged a bloody 30-year campaign against British rule in Northern Ireland.
The robbery, and a highly publicised murder in the city a month later, heaped pressure on the IRA to sever criminal links and ultimately spurred its July pledge to down arms for good.
The IRA denied any part in the raid.
Northern Bank withdrew its paper currency to thwart attempts to launder the cash, but police said up to 10 million pounds of the haul was untraceable. So far they have recovered only relatively small amounts of cash thought to be from the raid.
On Monday, a 42-year-old man was charged in a Belfast court with attempting to pervert course of justice by making false statements about the circumstances of the raid.
Martin McAliskey, a self-employed salesman from County Tyrone, was remanded on bail and will appear again in court on December 5.