The application of methyl jasmonate (MeJA) to grapes (Vitis vinifera L.) may decrease fruit detachment force (FDF) and promote the development of dry stem scars on the berries, both of which could improve the quality of machine-harvested raisin grapes. However, treatment with MeJA also promotes preharvest fruit drop, which is undesirable. Thus, experiments were conducted to determine how the concentration of MeJA applied and time after treatment affect FDF and abscission of grapes. Mature ‘Thompson Seedless’ grapevines were treated with one of five different solutions containing 0, 0.2, 2, 10, or 20 mm MeJA, and FDF and fruit abscission were monitored for ≈2 weeks. Treatment with 2 mm or less MeJA had inconsistent effects on FDF and did not promote abscission, whereas treatment with 10 to 20 mm MeJA reduced FDF within 2 to 3 days after treatment (DAT) and promoted abscission, which began on ≈3 DAT and persisted for ≈8 DAT. Thus, to optimize the use of MeJA as a harvest aid for ‘Thompson Seedless’ may require application of between 2 and 10 mm MeJA followed by harvest within 3 DAT.