"The tribe faces a suicide rate at least 34 times the national average due to the loss of their ancestral lands and constant attacks by gunmen," the London-based group said in a statement.
Guarani Indians, whose total population in Brazil is estimated at 46,000, have been trying to recover a small portion of their original territories, but face violent resistance from wealthy ranchers as well as soya and sugar cane plantation owners.
Survival wrote in a statement issued on the eve of issued on the eve of World Mental Health Day that, on average, at least one Guarani has committed suicide every week since the start of this century.
The group cited Brazilian health ministry data indicating that 56 Guarani Indians committed suicide last year, although it added that the actual figures are likely to be higher due to under-reporting.
"Most of the victims are between 15 and 29 years old, but the youngest recorded victim was just nine years old," Survival said.
"The Guarani are committing suicide because we have no land. We don't have space any more," Rosalino Ortiz, a Guarani tribesman, was quoted as saying.
"In the old days, we were free. Now we are no longer free."
Brazil's indigenous activists last week protested outside Congress in Brasilia against a constitutional reform that would transfer from the executive branch to legislators authority to approve and demarcate native lands and environmental conservation parks.
The Justice Ministry subsequently forwarded a legal finding to the House of Deputies, slamming the amendment.
"We believe this measure is not only ill-timed and inappropriate, but also unconstitutional," said Justice Minister Jose Eduardo Cardozo.
"The demarcation of Guarani land should have been completed many years ago, but the process has stalled," Survival said.
It urged the Brazilian government "to demarcate Guarani lands as a matter of urgency" and pressed US agribusiness giant Bunge and other such firms "to stop buying sugar cane from Guarani land."
Roughly 12 percent of Brazil's land currently is recognized as indigenous territory.
Native Indians grouped in 305 tribes represent less than 0.5 percent of the more than 200 million Brazilians.
World Mental Health Day, a United Nations-backed initiative, is held on October 10 each year to raise awareness about mental health issues worldwide.