Guttation is a common feature of cucumber leaves under high relative humidity conditions; however, little is known about the role of guttation in the transmission of Pseudomonas amygdali pv. lachrymans (Pal), which is the pathogen of cucumber angular leaf spot (ALS) disease. In this study, experimental evidence for the transmission of Pal inside cucumber plants and through guttation was provided, and the results proved that Pal can be transmitted from the bottom leaf to the upper leaves inside the plant and excreted from the upper leaves through guttation. After that, the third leaf of cucumber was inoculated with Pal bacterial suspension, Pal was detected on the fifth leaf, the petiole, and the stem and in guttation drops. Healthy cucumber seedlings were infected by Pal in the guttation droplets, indicating that guttation fluids containing Pal could become a potential source of secondary infection. The results from this study verified the hypothesis that guttation is a potential route for Pal excretion from cucumber plants and may be a source of secondary transmission under high relative humidity conditions.