Soft Muar Clay in Malaysia is of marine origin, and is characterized by a high water content that is close to or greater than its liquid limit. The variation of in situ vane shear strength is compared with various undrained laboratory tests conducted on undisturbed specimens recovered from different depths. The effects of the overburden pressure and Atterberg limits on the vane shear strength are discussed. Correlations between the field vane shear strength and the cone penetration resistance are also determined, and the effect of plasticity index on the normalized cone resistance is discussed. The results obtained for the Muar Clay are compared with other available data for selected soft marine clays. The use of the corrected and uncorrected vane strengths in the prediction of slip surface is also discussed.