What is the world's longest mountain range?

The Ocean Ridge (also known as Mid-ocean ridge), which runs more than 40,000 miles on the sea floor of five oceans is the longest mountain chain in the world. The longest mountain range on dry land is the Andes, which stretch 4300 miles through seven countries in South America.

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The mid oceanic ridges.

However, if you mean above the sea then the Rockies / Andes (if you count them together).