Sediment deposited in back-arc basins preserves a record of the extensional, volcanic and tectonic history of the arc-backarc systems. Back-arc sedimentation is of particular interest as seafloor massive sulfide deposits may be preserved in back-arc basin sediments. The study of back-arc sedimentation using acoustic data, such as high-resolution sub-bottom profiling data (Parasound) and seismic reflection data, can be a much more cost effective approach than analysis of sediments recovered from drill cores. In this study, we use these two acoustic datasets to build a facies model of sedimentation in the northeast Lau Basin, an actively opening back-arc basin in the southwest Pacific Ocean. Using 830 km of Parasound and 730 km of seismic lines along 4 transects of the Lau Basin, we constructed one of the most detailed models of sedimentation in a back-arc basin to date. Parasound data show distinct echoes with sub-bottom reflections indicative of a high proportion of hemipelagic sediment, whereas the indistinct echoes with few to no sub-bottom reflections indicate a higher proportion of coarse, bedded, volcaniclastic turbidites. Hyperbolic echoes are associated with regions of rugged or uneven terrain characterized by exposed, rough basement or deposits formed by contour currents, turbidity currents, slumps or slides. These relationships form the basis of an echo-facies legend developed for typical back-arc basin sediments. The echo-facies observed in the Parasound, and confirmed by deeper-penetrating seismic reflection data, provide important insights into the sedimentary processes involved in back-arc sedimentation. We observed mass transport deposits (MTDs) in all of the sub-basins and slope deposits within and on the flanks of active rifts (e.g., the Fonualei Rift and Spreading Centre, FRSC), suggesting a direct correlation between MTDs and zones of active rifting. We observed an overall increase in sediment thickness toward the Tofua Arc which suggests it is the main sediment source, but local variations in sediment thickness suggest significant input from local intrabasinal seamounts. The uppermost echo-facies in over 60% of the sub-basins in the study area is dominated by hemipelagic material, which suggests an abrupt transition in the dominant sediment source from volcaniclastic to hemipelagic at around 0.3 Ma, when a period of volcanic quiescence from the Tofua Arc began. The study shows that a near complete record of basin evolution can be constructed using geophysical and acoustic methods and that this work may help to locate future drill sites where in situ data can be collected.