No

A cyber attack is a cyber operation, whether offensive or defensive, that is reasonably expected to cause injury or death to persons or damage or destruction to objects.

Michael Schmitt et al; Tallinn Manual 1.0, pp 92
  • Cyber Operation?
  • Expected to cause death or injury to people or objects?
Titan Rain does not match this definition. Here's a list of actual cyberattacks and and why this matters.

What happened?

Chinese hackers infiltrated U.S. Department of Defense networks in an operation known as “Titan Rain.” They targeted U.S. defense contractors, Army Information Systems Engineering Command; the Defense Information Systems Agency; the Naval Ocean Systems Center; and the U.S. Army Space and Strategic Defense installation.