Is it possible to visit Blue Lagoon with an 11‑hour layover?

Yes, it is allowed, but you will have to go through security if you leave the airport. And I would think an eleven-hour layover would be enough to give you a few hours at the Blue Lagoon. Keflavík Airport itself is pretty small, so it doesn't take that long to get through security, either leaving or coming back. I would probably check a bag so that you just have to deal with a personal item, though. But if that's difficult, Blue Lagoon does have luggage storage, both for small bags and one outside the car park for larger ones. https://support.bluelagoon.com/hc/en-us/articles/360006000637-Storing-your-luggage

With regards to transport, the Blue Lagoon has a transport service that you can book to get to the lagoon from the airport and then back. I think it's around $35 to $40 each way, but that's probably your best option. You can find more information here: https://destinationbluelagoon.is/transport

Sky Lagoon, on the other hand, is farther away (near Reykjavik), and it would take longer to get there; probably an hour and a half with transfers. You could take the FlyBus into Reykjavik and then get a bus out to the lagoon, but it would take much more time, I would think, than just going to the Blue Lagoon. Also keep in mind that you would need to be back at the airport two hours ahead of time for security. I wouldn't try it myself but I'm not a risk taker when it comes to missing a flight.

And I just looked at the times that you will be there. The FlyBus into Reykjavik only runs till 11:00 p.m., and then doesn't start up again till 3:30 in the morning. So if you did go into Reykjavik for the Blue Lagoon, you would need to catch one of those late buses back before they stop running at 11, which limits your time. So I would stick with the Blue Lagoon.

In terms of recommending the experience, sure, if that's your only option and it's either that or sitting around a small airport for 11 hours. I didn't go on my first trip to Iceland because I was only there for 2 1/2 days and had a lot of other things that I wanted to do, but in your case, it makes sense.

I replied on the other forum you posted to.

That basically gives you 6 hours. Figure an hour to get out of KEF, then be there 3 hours before your next flight, thus the 7 hours. That is plenty of time to head into Reykjavik. You can store your bags at the airport and then take either the FlyBus or a taxi into town. FlyBus is roughly $39 pp each way, and taxis run about $150 each way. When we don't rent a car, we prefer taxis since the FB stops at the bus terminal and you change to a small bus, and my husband gets annoyed with that, lol.

If you head into Reykjavik it is about 45 minutes each way. You could head into town, so say arrive about 4pm, and then you have until 10pm. That would give you time to wander the city, have a nice dinner, go to Sky Lagoon and then head back to KEF. Another option instead of just doing dinner would be to do the food tour. That is excellent and we have done it a few times. It lasts 3 hours, and you get to try quite a few restaurants, so IMO this is a great quick visit thing to do. The Sky Lagoon does work with Reykjavik Excursions and you can get a transfer from sky to the central bus terminal, and then you can take the Flybus from there to KEF.

Mardee--The Flybus operates from KEF at all hours. They are there after every arriving flight, but the pickups from in Reykjavik back to KEF are only during "normal" hours.

Oh, good to know, mikliz. Thanks! That's a good idea about going into Reykjavik, and good to know about the FlyBus. You're much more familiar with Iceland than I am, so hopefully the OP will follow your advice.

Mardee--The FlyBus can be confusing. On our first trip years ago, there was another company that did this but you had to buy a timed ticket ahead of time and that was a mess sometimes. The FlyBus does make it easy with being able to just walk out the door and go to the bus. On one trip, we were on the bus waiting for it to fill up and the driver excitedly got up and told us to come with him. The aurora was out, even there at the airport with all the lights!

mizliz, I couldn't remember what I had taken when I was there in 2018, so I checked my files and they actually had two buses at that time: FlyBus and GrayLine. FlyBus was the most convenient, as it paid extra to be able to pull up right at the airport, whereas with Grayline you had to walk a few hundred feet more to get there.

But the price difference was considerable. FlyBus cost around $38 at the time, and GrayLine was only $22, so I opted for it, but evidently it's no longer in existence.

Mardee--It was Grayline that we used in 2015 as well. They were good then, but things change:)