Cole was immediately reinstated by the Lightning, who had suspended him with pay.
"I'm grateful for the opportunity to respond to and refute the anonymous allegations made against me, which I categorically deny," Cole said in a statement. "I'm looking forward to returning to the ice with my teammates and will have no further comments on this matter going forward."
An anonymous user posted a lengthy statement on Twitter on Oct. 7 accusing Cole, 33, of sexually abusing and grooming her as a minor. The person, who identified herself as "Emily Smith," said she initially met Cole while she was in high school and that they had a relationship lasting four years.
The league's investigation lasted a week and was conducted by the NHL security and legal departments. The NHL said it was unable to contact the source of the Twitter account.
The league said it did a detailed review of online and social media, public data and court records as well as law enforcement checks, plus interviews with Cole and "other individuals with potentially relevant information."
The NHLPA issued a statement saying that while it was "pleased" that the situation had been resolved, the union is is concerned about the precedent of suspending a player based on anonymous allegations.
"Players should never be subject to suspension or discipline in response to unsubstantiated and anonymous accusations," the NHLPA statement said. "Removing a player from his team under these circumstances is inappropriate and grossly unfair."
The NHLPA said it would talk with the league about how to address any similar situations in the future.