Phytoplankton, the primary producers of the marine food web and key agents in carbon sequestration, are critical to marine ecosystem function and climate regulation. Variability in the timing and abundance of phytoplankton blooms significantly impacts marine ecosystems, as the survival and recruitment of higher trophic organisms depend on food availability. In the Indian Ocean, biophysical interactions driven by the southwest and northeast monsoon systems lead to phytoplankton blooms. However, the basin has experienced sustained warming of 1.2{degree sign}C per century throughout the 20th and early 21st centuries due to anthropogenic greenhouse gas emissions, with projections indicating further warming. We examine the trends in phytoplankton phenology from 1998-2022 using remotely-sensed ocean color dataset. Our findings suggest a delaying of bloom onset by 25 days in the northern Indian Ocean, with blooms now occurring in July compared to June in earlier years. On the contrary, in the south tropical Indian Ocean, bloom onset has advanced by 58 days, shifting from June to May. Additionally, significant regional variations in trends are observed across the tropical Indian Ocean. The climatological atlas prepared as a by-product of this work provides a baseline to understand the phytoplankton bloom timings in the Indian Ocean during boreal summer and winter, and has implications for ecosystem management and conservation. Future work involves investigation into the underlying physical mechanisms driving these shifts in phytoplankton phenology to understand the changing biophysical interactions in a warming climate and to evaluate their broader ecological impacts.