Modern olive (Olea europaea L.) mill extraction techniques generate a large amount of two-phase olive mill waste (OMW) containing up to 90% organic matter. Recycling OMW as a soil amendment may represent a solution for its disposal, especially in Mediterranean areas, where many soils are characterized by very low organic matter levels. A 10-yr field study was conducted to evaluate the long-term sustainability of raw OMW disposal as a soil amendment on an olive grove in Elvas, Portugal. The soil was amended with OMW at rates of 0, 30, and 60 Mg ha−1, dry weight equivalent, for 8 yr, with cumulative and residual effects being assessed in the last year and 2 yr after the last application. Long-term applications of OMW to soil led to cumulative and residual effects on soil physical [aggregate stability and bulk density (Db)], chemical [total organic carbon (TOC) and its humified fractions, total N, available P, and K], and biochemical [dehydrogenase (DH), urease (UR), β-glucosidase (GLU), alkaline phosphatase (PHO), and arylsulfatase (ARS) activities] properties, and contributed to an increase in olive yields compared with the control (>17%, >1.47 Mg ha−1). However, electrical conductivity (EC) rose significantly with OMW application, especially in the residual year, ranging from 0.513 dS m−1 for the unamended soil to 2.29 dS m−1 at the 60 Mg ha−1 application rate. Repeated addition of OMW to olive grove soils may be a useful strategy for recycling this organic amendment to improve soil fertility and increase yields. However, long-term use of OMW increases the risk of soil salinity, especially if application rates are >30 Mg ha−1.