This article describes a laboratory testing program on a very soft, fine‐grained coastal sediment to investigate methods for characterizing both the physicochemical properties of the sediment and its stress‐strain‐strength properties in the laboratory. The very soft consistency of these coastal sediments presents a significant obstacle to obtaining valid mechanical properties from laboratory tests. Such properties are needed to develop constitutive models for interpretation of in‐situ tests, prediction of penetrating object behavior, and estimation of landslide and erosion potential. After physical and chemical characterization, the sediment from the Gulf of Mexico was tested using one‐dimensional consolidation and consolidated‐undrained triaxial and direct simple shear tests. Testing problems associated with the high compressibility and low strength of this material were identified and some solutions developed to mitigate their effects. The results indicate that, while better sampling procedures need to be developed, consistent consolidation and. stress‐strain‐strength parameters can be obtained. However, special testing considerations are vital, including innovative tube extrusion and specimen trimming procedures to minimize sample disturbance, high‐precision measurement instrumentation, and comprehensive error analysis to correct measured behavior. The article recommends additional research that is vital in order to reach more definitive and widely applicable conclusions about coastal sediment behavior.