Chevron was responsible for oil drilling contamination in a wide swathe of Ecuador's northern jungle and must pay nearly £6 billion in damages and clean-up costs, in what appears to be the highest damage award issued in an environmental lawsuit.
But the ruling by an Ecuadorean judge - nearly £5.4 billion plus a legally-mandated 10% reparations fee - was far below the £17 billion recommended by a court-appointed expert.
It also remains to be seen whether the plaintiffs - including indigenous groups who say their hunting and fishing grounds in Amazon River headwaters were decimated by toxic wastewater that also raised cancer rates - can collect.
Chevron, which earned nearly £12 billion last year, called the decision "illegitimate and unenforceable" and said it would appeal. It has long contended it could never get a fair trial in Ecuador and has removed all assets from the politically volatile Andean country, whose left-wing president Rafael Correa had voiced support for the plaintiffs.
The marathon, high-stakes case, fraught with corporate espionage and geopolitical intrigue, has been winding its way through US and Ecuadorean courts for more than 17 years.
Even Hollywood had a role, with Chevron successfully forcing documentary filmmaker Joe Berlinger to surrender out-takes from his documentary, Crude, about the dispute, a decision upheld by a US appeal court.
Those out-takes were used in an attempt to show that a plaintiffs' lawyer, Steven Donziger, had both denigrated and unethically influenced Ecuadorean justice.
The plaintiffs' lead lawyer, Pablo Fajardo, called the 187-page judgment "a great step that we have made toward the crystallisation of justice", but added that "we are not completely satisfied" with court-specified damage award. He said the plaintiffs were discussing whether to appeal.
The lawsuit was originally filed in a New York federal court in 1993 against Texaco and dismissed three years later after the oil company argued that Ecuador was the proper venue to hear the case. Chevron bought Texaco in 2001 and the lawsuit was refiled in Ecuador two years later.
Though it had only 47 named plaintiffs, the lawsuit sought damages on behalf of 30,000 people for environmental contamination and illnesses that allegedly resulted from Texaco's operation of an oil consortium from 1972 to 1990 in a Rhode Island-sized oil patch dug out of virgin rainforest.