All scheduled departures up to 31 December have been cancelled.
Impacted guests will be offered a 125% future cruise credit.
Alternatively, they can also opt – no later than 24 August – for a full refund of all monies paid to date.
"If itinerary changes are required, impacted guests and their travel agents are notified directly by Viking guest services," said the line in a statement.
Viking claims to have been the first cruise line to temporarily suspend operations due to coronavirus when it ceased sailing on 11 March.
In a letter to guests, Chair Torstein Hagen it was "in that spirit" the line had come to the decision to suspend operations longer-term.
"We are extending our temporary suspension of operations through 31 December, at which time we believe the world will be in a better position, and international travel will be less complicated," he said.
"On 11 March, when we became the first cruise line to temporarily suspend operations at the start of the Covid-19 pandemic, I would not have imagined that in August, we would still not be sailing and that I would be writing this letter to you.
"Every day, I am encouraged by the scientific advancements toward Covid-19 therapeutics and a vaccine. But as you well know, recent events have shown us the recovery from this pandemic will be sporadic, and the ability to travel freely across borders remains some time away.
"As keen as we may be to get back to exploring, for now, international travel must wait."
Hagen said Viking was committed to "maintaining the safest and healthiest fleet in the industry", stressing its focus on small but spacious ships would provide a safer environment when operations resume "than mega liners".
He said the line was finalising additional health and cleaning protocols, developed in collaboration with "leading international experts".
Hagen also confirmed the line was committed to expanding its fleet with the launch next year of its seventh ocean ship, Viking Venus, and its new Mekong vessel, Viking Saigon.
Then in 2022, the line will welcome its new expedition vessels, Viking Octantis and Viking Polaris, as well as its eighth ocean ship, Viking Mars, and first US river ship, Viking Mississippi.
Viking last week celebrated its 23rd anniversary, and Hagen said while the company was "very different today", its commitment to the safety and welfare of guests and crew was unwavering.
"I have said before that we will only sail again when it is safe to do so," he said. "We are a private, closely-held company, which means we do not have to rush the decision to return to service."
Options for guests and agents
Any guests whose voyage has been affected by the suspension will be offered a future cruise credit worth 125% of the monies paid to Viking, plus an additional 25%.
Viking said the added value would allow guests to switch their booking to a different season, choose a different itinerary, add pre/post-cruise extensions, upgrade their stateroom, or upgrade their pre-cruise travel.
Credit is fully transferrable to another person, and can be applied over the next 24 months to any Viking river, ocean, or expedition cruise. Anyone unable to use their voucher by its expiration date can return any remaining credit and receive a refund.
Anyone wishing to claim a full cash refund instead of receiving future cruise credit should contact Viking before 24 August.
