<p>Forest ecosystems account for about one-third of the Earth’s land area, and monitoring their structure and dynamics is essential for understanding the land’s carbon cycle and its role in the greenhouse gas balance. In this framework, process-based forest models (PBFMs) allow studying, monitoring and predicting forest growth and dynamics, capturing spatial and temporal patterns of carbon fluxes and stocks. The ‘Three Dimensional-Coupled Model Carbon Cycle—Forest Ecosystem Module’ (3D-CMCC-FEM) is a well-known eco-physiological, biogeochemical, biophysical process-based model, able to simulate energy, carbon, water and nitrogen fluxes and their allocation in homogeneous and heterogenous forest ecosystem. The model is specifically designed to represent forest stands, from simple ones to those with complex structures, involving several cohorts competing for light and other resources in a prognostic way. The model is also designed to simulate current forest management practices commonly applied in Europe. The 3D-CMCC-FEM model is implemented in C-language, which can be challenging for the broad public to use, thus limiting its applications. In this paper, we present the open-source R package ‘R3DFEM’ which introduces efficient methods for: i) generating and handling input data needed for the model initialization; ii) running model simulations with different setup and exploring input; and iii) plotting output data. The functions in the R-package are designed to be user-friendly and intended for all R users with little to advanced coding skills, who aim to perform simulations using the 3D-CMCC-FEM. Here we present the package and its functionalities using some real case studies and model applications.</p>