The ITC had widely been expected to side with Chicago-based Boeing, the world's largest maker of jetliners, which accused Bombardier of dumping the planes, or selling them below cost, in the US market.
Bombardier called the ruling a "victory for innovation, competition and the rule of law".
In a statement the company added: "The C-Series' development and production represent thousands of jobs in the US, Canada and the UK. With this matter behind us, we look forward to delivering the C-Series to the US market so that American airlines and the public can enjoy the many benefits of this remarkable aircraft."
Union Unite said Bombardier staff in Northern Ireland were "breathing a massive sigh of relief that the ITC had seen through Boeing's baseless complaint".
Steve Turner, Unite assistant general secretary, added: "The C-Series is a world-beating aircraft made by world-class workers. There can be no backsliding from the US government on this decision."
The trade dispute was highly political. President Donald Trump met with Prime Minister Theresa May at the Davos conference earlier this week and spoke about the special relationship between the two nations after a one-to-one meeting, where the Bombardier dispute was discussed.
Afterwards Trump said he had a "great relationship" with May and that trade would "increase many times".
The row over Bombardier stemmed from a complaint US aerospace giant Boeing made to the US Commerce Department last spring. Boeing claimed that Bombardier - which is based in Canada - received illegal state subsidies from Quebec and the UK. It was only because of these that Bombardier was able to sell the C-Series jets at US$19.6 million ($26.7m) each to Delta, hugely below their US$80m list price.
Calling the discounted price "absurdly low", Boeing accused Bombardier of trade dumping and lobbied US authorities for action. Boeing also claimed the Delta deal harmed sales of the smallest versions of its best-selling 737 jet, which could have competed with the C-Series for the Delta contract.
The US giant said the dispute was "not political, but legal" involving "a textbook case of dumping", adding that it "welcomes competition".
Bombardier refuted the claims, saying subsidies to support the development of new airliners are common around the world, along with heavy discounts for the first sales of new types. It also claims that Boeing does not produce an aircraft of the same size that could compete for the Delta deal.
Reacting to the ruling, Boeing said it was "disappointed the ITC did not recognise the harm that Boeing has suffered from the billions of dollars in illegal government subsidies that the Department of Commerce found Bombardier received and used to dump aircraft in the US."
The US aerospace giant said it was "feeling the effects of those unfair business practices in the market every day" and warned it would not "stand by as Bombardier's illegal business practices continue to harm American workers and the aerospace industry they support".
British aerospace and defence trade group ADS said it welcomed the decision, adding that "aerospace is a competitive and growing sector, and it is important that customers should be given access to the best technology and the most innovative products. It is great to see the ITC has upheld the evidence put forward by Bombardier and by governments in the UK and Canada."
The row has drawn in politicians, with British MPs arguing for multi-billion pound defence deals with Boeing to be cancelled in retaliation. Canadian Prime Minister Justin Trudeau had taken a much harder line, saying his country would not do business "with a company that is trying to sue us, eliminate tens of thousands of jobs and put our companies out of business".
