The first mystery sailing to be run by Uniworld Boutique River Cruises sold out in 48 hours – and a second released a fortnight later in 24 hours.
The ten-day itineraries in Europe depart on June 12 and September 23, 2022 respectively.
Both will feature “over-the-top, unique experiences” both on and off the ship.
All the details – including the destinations – will remain a surprise to passengers until the last possible moment.
The second mystery cruise will travel to completely different destinations from the first departure.
Passengers will receive a packing list in advance, but the big reveal will only take place when they are already en route to the airport.
UK and Ireland managing director Chris Townson said: “We’ve over the moon at the response to our mystery cruises, which is a sign of the incredible support and loyalty we are lucky enough to receive from our guests, and for which we are very grateful.
“The question we’re being asked of course is, will we announce another mystery cruise?
“I think we’re still figuring that one out – it takes a huge amount of work to curate these thrilling, one-of-a-kind adventures filled with experiences we’ve never offered before, and to put the systems in place to ensure that they remain a closely guarded secret.
“Certainly, we are delighted at the way they have been received, and we will have more exciting announcements down the line as the world opens up to travel once more.”