The Norwegian company said neither side was able to agree on the final details of a deal to form Exmar LNG.
The tie-up was announced on 1 July this year and would have involved a combination of LNG assets.
But the merger was subject to "satisfactory due diligence" and "agreement on definitive transaction documents", Flex LNG said.
"The parties have failed to agree on the definitive transaction documents and the previously announced transaction will not be completed," it added.
