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The G protein α-subunit, GPA1, is an integral component of several signaling pathways in plants, including response to abiotic stress. However, the molecular mechanism behind these processes remains largely unknown in the cucumber plant (Cucumis sativus L.). In order to further understand the role of CsGPA1 in cucumber under drought stress, changes in plant growth, physiological parameters, and gene expression of CsAQPs were all measured under water stress induced by polyethylene glycol (PEG) using wild type (WT) and CsGPA1-interference (RNAi) cucumber seedlings. Our results demonstrated that the RNAi plants had lower drought tolerance, displaying seriously withered leaves, lower relative growth rate, lower root-shoot ratio, and lower root activity under drought stress compared to WT plants. Physiological studies indicated that the suppression of CsGPA1 weakened drought stress tolerance due to higher water loss rate in the leaves, higher levels of hydrogen peroxide (H2O2), increased malondialdehyde (MDA) content, lower free proline content, lower soluble sugar content, lower soluble protein content, and decreased antioxidant enzyme activities. qRT-PCR analysis demonstrated that the interference of CsGPA1 up-regulated the expression of most AQP genes (except for CsPIP2;3 in leaves) and down-regulated the expression of CsPIP1;2, CsPIP1;4, CsPIP2;1, and CsPIP2;4 in roots under drought stress when compared to WT plants. Our results demonstrated that CsGPA1 could function as a positive regulator in drought stress response by decreasing the accumulation of reactive oxygen species (ROS), improving permeable potentials, and reducing water loss in cucumber plants.