Einar Sparboe Lysnes cautioned in a statement on Thursday that in the last few months the situation had reached a crisis point.
"We have several times discovered that there were no free spaces, and so simply had to let them back out," he told VG newspaper. "As a result new criminal offences take place that otherwise would not have happened. Our ability to prevent crime and ensure safety is reduced."
Lysnes' warning followed a complaint earlier this month from the head of the police in Vestfold county, near Oslo, who proposed freeing up space by converting old army camps in far-northern Norway into emergency prisons for foreign criminals.
Lysnes said that on Thursday, the day he made the problem public, he had been forced to release a 20-year-old repeat offender, who had been charged this time with three serious thefts, six burglaries and fraud.
Earlier in the autumn, he was forced to release a car thief who only a few weeks later went on to carry out further car thefts, until he was finally stopped for driving a vehicle when drunk.
Jorid Midtlyng, who heads the prison services in Norway's northern region, admitted that prisons in the region were at full capacity.
"We apologize for the situation, but we are jam-packed," he said. "We do everything we can… yet it does occasionally happen that we can't offer places."