Researchers at the University of Washington in Seattle, working under contract to the biotechnology giant, used a gene-sequencing technique to map roughly 85% of the grain's genetic code.
Scientists from the world's largest rice producer, China, said the research could enable Chinese farmers to plant crops with much higher yields in four to five years.
Western farmers have cut back on genetically-modified (GM) crops because of consumer resistance, but in China the need to ensure food production keeps pace with population growth has tended to outweigh these worries.
Rice is a staple diet for 40% of the world's population.
Grass research
Researchers managed to decode all 12 chromosomes of rice. The results will form the basis for study of other grass species such as corn, sorghum and wheat.
"When we decipher the rice genome, we will have the equivalent of an encyclopaedia that contains the instructions for creating a life form," Leroy Hood, head of the laboratory at the University of Washington, said.
The breakthrough is expected to enable scientists to engineer genetically-modified rice strains resistant to drought, salinity and insects.
Monsanto said it would make the research public through the International Rice Genome Sequence Project (IRGSP), an international research consortium of 10 research teams working to complete the sequencing of the rice genome.
Other researchers will also have access via the internet, although Monsanto retains the right to licence any developments based on its own research.
The move is a bid to improve the agro-chemical giant's image following bad publicity over its sterile seed technologies carrying the "terminator" gene.
Food security
Global rice production last year was estimated at well over 500 million tonnes, 92% of which came from Asia.
"This technology is very important for food security, especially for places like China, India and Africa," Chen Zhangliang, Vice-President of Peking University, said.
China supports a fifth of the world's population with only 7% of the world's arable land.
The Monsanto announcement came as Joseph Estrada, President of the Philippines, launched an attack on Western countries for neglecting rice research.
In a speech at the 40th anniversary of the International Rice Research Institute near Manila, President Estrada said the fruits of scientific advances were not trickling down to poorer nations.
Some aid organisations have attacked the emphasis on GM technology, calling it a "technical fix" that does little to address the real social and economic causes of world poverty and hunger.