Forest Farmers' Organizations (FFOs) are gaining scholarly traction as a means to mobilize non-industrial private farmers to upscale tree-growing practices, thereby contributing to securing the increasing demand for wood products. However, how farmers perceive FFOs and what drives their willingness to participate in FFOs are not yet fully understood. Using two case studies, this study aimed to examine smallholder tree growers' perceptions of FFOs and the main factors influencing their willingness to participate in FFOs. Data were collected from 185 and 190 farm forestry households in Ethiopia and Tanzania, respectively, and triangulated with focus group discussions and key informant interviews. Grounded in the social exchange theory, we employed regression analysis and thematic analysis to analyze the data. The results showed that 74% and 90% of smallholders would be willing to participate in an FFO in the Ethiopian and Tanzanian cases, respectively. In Ethiopia, farmers understood an FFO as a tree marketing cooperative or a collective wood marketing enterprise. Household heads in the middle age group and those who experienced price satisfaction in the last woodlot sales would be more likely to participate in an FFO. In Tanzania, tree growers associations focused on social learning and wood production improvement. Those more likely to participate had larger household sizes and difficulty accessing the wood market. We proposed a hypothesis and further research agenda, calling for a panel study to complement our conceptual assertion.