UC3 Nautilus was a privately built Danish midget submarine. It was built over a three-year period by Peter Madsen and a group of volunteers, and cost approximately US$200,000 to build. The submarine was Madsen's third submarine design.

Peter Langkjær Madsen is a Danish convicted murderer and former entrepreneur. In April 2018, he was convicted of the August 2017 murder of Swedish journalist Kim Wall on board his submarine, UC3 Nautilus, and sentenced to life imprisonment.