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.

The murder of Kim Wall, known in Denmark as Ubådssagen, took place on 10 August 2017, after Swedish freelance journalist Kim Wall boarded the midget submarine UC3 Nautilus, in Køge Bugt, Denmark, with the intent of interviewing its owner, Danish entrepreneur Peter Madsen.