The U.S. International Trade Commission (ITC) ruled Friday that HTC's smartphones had violated two of Apple's patents on wireless technology.
The decision sent HTC's stock down nearly 4 percent in Monday trading on the Taiwan Stock Exchange.
The company plans to appeal the case.
"We are highly confident we have a strong case for the ITC appeals process and are fully prepared to defend ourselves using all means possible," said HTC General Counsel Grace Lei in a statement. "We strongly believe we have alternate solutions in place for the issues raised by Apple."
HTC said it had yet to review the judge's full opinion and did not state which two patents were covered by the judgement, but the Wall Street Journal's AllThingsD tech blog reports that the ruling covered patents numbered 5,946,647 and 6,343,263.
If HTC was found to have violated the '647 patent, it could spell bad news for Android, since the patent appears to relate to the open-source operating system, which is used in many smartphones manufactured by HTC. The ruling could also have implications for other companies that manufacture Android-based smartphones.
The judge ruled against HTC despite a recommendation in April from the ITC's staff attorney that the handset manufacturer was not in violation of the patents.
The decision came less than a week after Apple filed its second ITC complaint against HTC over patents used in the company's smartphones. The ITC has yet to announce whether it will investigate the claims.
Apple first sued HTC in March of last year over 10 patents. HTC has filed a counterclaim against Apple, which is currently being investigated by the ITC. A ruling on HTC's countersuit is expected in September.