<p>This study investigated the spatiotemporal heterogeneity of riparian vegetation and phytoplankton communities and their environmental drivers in urban rivers of Xi’an, where riparian vegetation supports pollution control and hydrological regulation, and phytoplankton underpin water quality and energy cycling in aquatic ecosystems. Surveys were conducted in June, September, and November 2022 across major rivers in Xi’an, analyzing community structures of both riparian vegetation and phytoplankton while measuring physicochemical water properties, habitat quality, social factors, and riparian soil heavy metals. In total, 47 plant species (mainly ferns and angiosperms, spanning 2 phyla and 13 orders) and 235 phytoplankton taxa (predominantly diatoms and chlorophytes, from 7 phyla) were identified. The ecological risk index (RI) and pollution load index (PLI) indicated the highest ecological risk and pollution levels during the dry season. Canonical correspondence analysis (CCA) showed that Zn and Pb significantly influenced riparian vegetation, whereas Cr and DO were key drivers of phytoplankton cell density in the dry season. A generalized linear model (GLM) revealed a seasonal shift in key environmental drivers for both communities—from stressors and pollutants toward resource limitation—governed primarily by hydrological changes. These results clarify differentiated seasonal response patterns of riparian and phytoplankton communities to environmental factors, providing evidence to inform ecological restoration and management of urban rivers. The observed seasonal driver transitions may reflect broader hydrological–ecological mechanisms applicable to urban waterways in similar climatic regions.</p>