It happened just hours after crisis talks were held into an earlier attack this week, which disrupted internet traffic for some of the company's five million customers around the country.
Telia, which is part-owned by the state, told news agency TT that the latest attack, which started after 1 a.m. was similar to the one earlier in the week.
The head of operations at Telia, Peter Kjellberg, says it's hard to know how many customers are affected.
"It is difficult to say at present. We rely on input from customers to be able to say how bad it is, but it's a big problem."
He says that the hacker attack is directed against one of Telia's business customers.
"It is an attack against another company that goes through our servers, and it means that our servers cannot handle the pressure," he tells Swedish Radio News.
This is the second time in a week, what can you say about it?
"We are tereating this very seriously and have launched emergency meetings and also investigations in several places to find out what we can do to avoid this situation in the future."
Over five million Swedes use Telia for essential services including telephone, TV and mobile connections.