The defendants, all prominent scientists or geological and disaster experts, were sentenced to six years in prison.
Earthquake experts worldwide decried the trial as ridiculous, contending there was no way of knowing that a flurry of tremors would lead to a deadly quake.
"It's a sad day for science," said seismologist Susan Hough, of the U.S. Geological Survey in Pasadena, Calif. "It's unsettling."
That fellow seismic experts in Italy were singled out in the case "hits you in the gut," she said.
In Italy, convictions aren't definitive until after at least one appeal, so it was unlikely any of the defendants would face jail immediately.
Italian officials and experts have been prosecuted for quake-triggered damage in the past, including a 2002 school collapse in southern Italy that killed 27 children and a teacher. But that case centered on allegations of shoddy construction in quake-prone areas.
Among those convicted Monday were some of Italy's best known and most internationally respected seismologists and geological experts, including Enzo Boschi, former head of the National Institute of Geophysics and Volcanology.
"I am dejected, desperate," Boschi said. "I thought I would have been acquitted. I still don't understand what I was convicted of."
The trial began in September 2011 in this Apennine town, whose devastated historic center is still largely deserted.
The defendants were accused of giving "inexact, incomplete and contradictory information" about whether small tremors felt by L'Aquila residents in the weeks and months before the April 6, 2009, quake should have been grounds for a warning.
The 6.3-magnitude temblor killed 308 people in and around the medieval town and forced survivors to live in tent camps for months.
Many much smaller tremors had rattled the area in the previous months, causing frightened people to wonder if they should evacuate.
"I consider myself innocent before God and men," said another convicted defendant, Bernardo De Bernardinis, a former official of the national Civil Protection Agency.
Prosecutors had sought convictions and four-year sentences during the trial. They argued that the L'Aquila disaster was tantamount to "monumental negligence," and cited the devastation wrought in 2005 when levees failed to protect New Orleans during Hurricane Katrina.
Relatives of some who perished in the 2009 quake said justice had been done. Ilaria Carosi, sister of one of the victims, told Italian state TV that public officials must be held responsible "for taking their job lightly."
The world's largest multidisciplinary science society, the American Association for the Advancement of Science, condemned the charges, verdict and sentencing as a complete misunderstanding about the science behind earthquake probabilities.
There are swarms of seismic activity regularly in Italy and most do not end up causing dangerous earthquakes, said geologist Brooks Hanson, deputy editor of the organization's Science magazine. He said that if seismologists had to warn of a quake with every series of tremors, there would be too many false alarms and panic.
"With earthquakes we just don't know," Hanson said Monday. "We just don't know how a swarm will proceed."
Quake scientist Maria Beatrice Magnani, who followed the trial closely and knows the defendants professionally, called the outcome "pretty shocking."
She disagreed with putting scientists on trial, and contended that the death toll would have been lower had buildings in the quake-prone area been better reinforced.
The verdict left Magnani and others in the field wondering about the way they articulate their work.
"We need to be extremely careful about what we say, and the information we provide has to be precise. We cannot allow ourselves to slip," said Magnani, an associate research professor at the University of Memphis.
Comments on Twitter about the verdict abounded, with references to Galileo, the Italian scientist who was tried as a heretic in 1633 for his contention that the Earth revolved around the sun and not vice versa as Roman Catholic Church teaching then held. In 1992, Pope John Paul II declared that the church had erred in its ruling against the astronomer.
Still, some experts argued that the trial was about communicating risk and not about whether scientists can or cannot predict earthquakes.
"This was about how they communicated" with a frightened public, said David Ropeik, a risk communications consultant who teaches at Harvard and offered advice to one defendant scientist. It was "not Galileo redux," he said.
Defense lawyer Filippo Dinacci predicted that the L'Aquila court's verdict would have a chilling effect on officials tasked with protecting Italians in natural disasters. Public officials would be afraid to "do anything," Dinacci told reporters.