The diversity, drivers and consequences of vegetation seasonal dynamics in tropical forests are poorly understood. This is due to the tremendous diversity of phenological behaviors across, but also within plant species. Documenting several crucial aspects of seasonal biological variation, such as leaf renewal, area and ageing, needs dedicated measurement protocols, which remain barely implemented across Amazonia (crypto‐phenology). Such information is however critical for understanding and predicting the influence of environmental drivers and climate change on forest functioning at any meaningful scale. As an illustration, Dynamic Global Vegetation Models (DGVMs) have been so far limited in their ability to capture the seasonality of fluxes in tropical forests based on mechanistic underpinnings, hindering robust predictions of earth climate dynamics. The monitoring of tropical forest dynamics and productivity mostly rests on permanent field setups, such as flux towers census plot networks or litter traps, which are limited in terms of spatial and/or temporal coverage as well as monitored processes. A broader coverage is offered by satellite remote sensing products, although current atmospheric and directional corrections have been criticized. This project aims to lay the groundwork for a phenological observatory in French Guiana from which tree phenological strategies will be documented, crucial but hidden processes impacting forest fluxes (e.g. tree leaf area and ageing) will be measured, essential climatic drivers of seasonal dynamics will be identified, and local processes will be up‐scaled to stand and regional scales. In relation to existing networks (SOERE TEMPO, ICOS) and building on their standardized protocols, PhenObs will account for the complexity and specificity of tropical forests. Thanks to the dynamism and proven collaborative experience of the CEBA community, and long‐established field stations, French Guiana is the right place to undertake the ambitious multidisciplinary effort required to study transversal aspects of phenology in a tropical context. For the most intensive measurements, efforts will be focused on existing flux towers (Nouraflux and Guyaflux) and surrounding permanent plots, notably with the help of dronebased surveys. Other approaches, based on citizen science and the use of scanned herbarium samples will also be developed to broaden our spatial and temporal sampling. Upscaling to the regional scale will then be performed using satellite imagery and vegetation/forest models.