Council leader Glen Sanderson said that although it was disappointing, the decision by the Government would help protect the most vulnerable in the county.
He said: “I’d like to thank everyone once again for doing their bit and sticking to the rules over the past few weeks.
“We know lockdown has been tough on all of us, on our families and on our businesses, but by working through this together we are starting to see results – our rates are now going in the right direction although they are still too high.
“And while it’s disappointing the region finds itself going into Tier 3 we are determined to do everything we can to move out of it as quickly and safely as we can.
“Of course, the reopening of shops and retail is very welcome news for our residents and businesses in that sector, and we know this will result in busy town centres as people head out for their Christmas shopping.
“It's important therefore that we support our local independent businesses, and we’re actively promoting Shop Local, and we'll work hard to encourage everyone to follow the guidance and stick to hands, face, space at all times.
"And we’ll continue to offer support to those who need it through Northumberland Communities Together, as well as promote our business grant support schemes to our hard-hit businesses."
The Northumberland Gazette has also been campaigning for people to #supportlocal.
Director of Public Health Liz Morgan added: “Infection rates in Northumberland have dropped considerably over the last 10 days but they are still very high.
“It does feel like we are getting things more under control though, but this will only be possible if we all continue take responsibility and do our bit for one another.
“It's also important to remember that moving into the Very High Alert category is not just about falling cases, it's more complex than that.
“We have to look at the rates in all age groups, particularly older people who are more vulnerable to the virus; and we have to take into consideration the pressure on our local hospitals and NHS services."