Tropical rainfall exhibits a prominent annual cycle, with characteristic amplitude and phase representing the range between wet and dry seasons and their onset timing, respectively. Previous studies note enhanced amplitude over ocean and delayed phase over land in model projections of global warming, underpinned by first-order physical principles. However, it is unclear whether these changes have emerged in observations. Here we use gridded precipitation datasets to report a seasonal delay of 4.1 ± 1.1 and 4.2 ± 0.9 days (P < 0.05) during 1979–2019 over the northern tropical land and Sahel, respectively. Most of the delay is driven by external forcings, dominated by greenhouse gases (GHG) and anthropogenic aerosols (AER). Increasing GHG and decreasing AER in the recent decades delay rainfall by producing a moister atmosphere, thus increasing its lag in response to seasonal solar forcing. As GHG increase and AER decrease, these seasonal delays are projected to further amplify in the future.