Recently, the Lebanese wine sector has been witnessing a non-precedent growthproducing huge amounts of winery wastes referred to as grape marc. The effect ofusing grape marc compost on lettuce (Lactuca sativa L.) production wasinvestigated in an open-field experiment in Central Bekaa. Seedlings of theRomaine variety were planted in different substrates: S1: 100% soil or control, S2:75%soil + 25% grape marc compost, S3: 25%soil + 75% grape marc compost andS4: 100% grape marc compost. Root growth, leaf growth and leaf characteristicswere compared among the different mixtures. Tests showed that the grape marccompost contained acceptable values of nitrogen, phosphorus, potassium, organicmatter and a perfect germination index ranking between 0.8 and 1 for the direct anddiluted solutions. Results showed that at early stages of growth, the best resultswere obtained from plants grown in the substrate S4. However, at later stages ofgrowth, grape marc compost with a percentage higher than 50% (S4) in the mixtureinduced lower averages of leaf number, length and width of largest leaf and leafweight and those higher than 25% (S3 and S4) increased dry matter and totalsoluble solids content due to its low water holding capacity causing a water stresson plants. Root growth was proportional to increased percentages of grape marccompost. Finally, composted grape marc provided the highest benefit to plantswhen added to soil in quantities of 25% by volume allowing the best yield increase(47%) compared to control.