Construction of civil engineering structures on weak soil without taking necessary precautions may be risky. Alluvial soil that has not completed its geological formation has a high void ratio and contains organic material therefore, the strength properties of these soils should be examined carefully. In this study, the undrained shear strength (cu) behavior of natural and polypropylene (PP) fiber-reinforced alluvial clays was investigated with a laboratory Vane shear test. To examine the moisture content effects on cu behavior of alluvial clay, samples were prepared in 0.50 liquid limit (LL), 0.75 LL, and LL water contents. The PP fibers used were 6 and 18 mm long, they mixed with soil 0.1, 0.5, and 1% by dry weight of the sample. The Vane shear tests were performed at two different depths to investigate the overburden pressure effect. The increase in water content caused a significant decrease in cu. The laboratory results indicated that the cu of PP reinforced (1% and 18 mm PP) alluvial clay deposits prepared in 0.5LL, 0.75LL, and LL water contents were 56.6, 20.7, and 8.4 kPa, respectively. The increase in PP fiber content increased the cu of alluvial clay deposits. The length of fiber was directly proportional to cu values. The effect of fiber was more pronounced in long fiber added samples. The cu of natural and 1% fiber reinforced (6 mm and 18 mm) samples prepared in the same water content were 27.4, 29.1, and 55.7 kPa. The cu increased with increasing penetration depth.