The present invention relates to a device for the heat insulation of at least one underwater pipe (1) intended to be laid on the sea bed at great depth, comprising an insulating coating surrounding the latter and a protective envelope (3), characterized in that said insulating coating comprises a virtually incompressible liquid/solid phase change material (4) with a melting temperature T₀ higher than that T₂ of the medium surrounding the pipe in operation and less than that T₁ of the effluents circulating in the pipe, and said material (4) preferably being impregnated in an absorbent matrix (2) surrounding the pipe (1), preferably nearest its outer surface, which protective envelope (3) is resistant and deformable and ensures a containment against and about said insulating coating.