As part of the change, the company's New York Stock Exchange ticker symbol switched to "ACT" from "WPI."
Watson agreed to buy Actavis Group in April for about $5.6 billion. It completed the deal on Oct. 31 and announced the planned name change on that date. The new Actavis is the third-largest generic drug company in the world and has about $8 billion in annual revenue.
Actavis shares rose 86 cents to $86.59 in morning trading. The stock has risen 46.5 percent since rumors about Watson buying Actavis began to circulate in March. The shares have been trading at all-time highs for months, peaking at $91.47 in late December.