You don't believe it? Think about French Guiana! They share 673 km (418 miles) of their border which is 50 km more than with Spain.

However, considering Adélie Land, the longest border is shared with Australia: 2100 km (1305 miles). This is subject to discussion as Adélie Land is not actually a French departement and as the Sovereignty over the Antartic continent is particular.

Note: some sources indicate 730 km (454 miles), which does not contradict this DYK.