Arthropod distributions across riparian gradients remain incompletely understood. Within riparian buffers, invasive plants such as Fallopia japonica (Japanese knotweed) may influence abiotic conditions and potentially arthropod distributions. We quantified ground spider and ground beetle activity across a Japanese knotweed-dominated riparian buffer using five months of pitfall trapping data. Results from pitfall samples suggest that spider and beetle activity is negatively correlated with the presence of Japanese knotweed along the riparian gradient. We found that beetle activity declined with increasing distance from the river, while spider activity showed the opposite trend. Spiders and beetles also differed in the timing of their activity. Overall nocturnal beetle activity was much higher than diurnal beetle or spider activity. Ground spider and beetle activity levels were lower across the knotweed-dominated riparian area, which may be attributed to reduced litter and herbaceous vegetation, reduced light penetration, drier soil, lower plant diversity, or reduced prey availability in this area.