In an annual report, the FAO warned that a worst-case scenario of high-impact climate change would pound the communities that rely on agriculture for their livelihood, and food insecurity could spread to all regions.
It called for a "broad-based transformation of food and agricultural systems" to adapt to a warmer world, with an emphasis on supporting small shareholders.
"There is no doubt climate change affects food security," FAO chief Jose Graziano da Silva said.
Farming is both a driver of climate change, responsible for some 21 percent of global greenhouse gas production, and a victim, with crops adversely affected by drought and floods.
Adopting "climate-smart" practices, like planting nitrogen-efficient and heat-tolerant crops, or finding better ways to conserve water, would reduce undernourishment for many millions, the FAO said.
The report also called on signatories to the 2015 Paris climate deal to "put commitments into action", underscoring the need to help developing countries with climate change mitigation.