A provision in New York state's new gun-control laws that requires mental-health professionals to report potentially dangerous patients is drawing concern from experts who fear it could discourage people from seeking treatment and treads on client confidentiality.

The measure requires physicians, psychologists, nurses or clinical social workers to alert local health officials if a patient "is likely to engage in conduct that would result in serious harm to self or others." After an evaluation, the health officials would pass on the person's information to law-enforcement agencies that would be authorized to seize any firearm owned by the patient.

If a person is found not to own any firearms, the patient would be added to a statewide criminal background check database, marking a significant expansion of who would be made ineligible to legally buy a firearm.

"I understand the intent, but I fear these kinds of practices may well deter people from seeking care or fully disclosing" their condition to psychologists, said Harvey Rosenthal, executive director of the New York Association of Psychiatric Rehabilitation Services.

He said the legislation seems designed to allay public fears of guns in the hands of the mentally ill with little thought given to "the chilling effects" it may have on treatment. Studies have shown that mentally ill people are far more likely to be the victims of violence rather than perpetrators.

The prospect of being put on such a list could be enough to prevent someone from seeking care and creates the impression that "you're turning psychologists into police officers," said Eric Neblung, a psychologist and the president of the New York State Psychological Association.

Patients might worry about being honest: Even the most fantastical violent expressions could be subject to a legal mandate, Dr. Neblung said.

But DJ Jaffe, executive director of Mental Illness Policy Org, an advocacy group, said concerns over the legislation are "hyper-theoretical" and that the greater, and more practical, risk is that very seriously mentally ill people "have a plan to kill people, and it doesn't go reported."

Mr. Jaffe said the legislation should go further by using the names compiled under the law to help those people get "priority treatment."

Mental-health professionals already have an ethical obligation to respond to threats of violence, such as warning potential victims of possible threats. Dr. Neblung said the new law compels caregivers to break confidentiality before clinical remedies are exhausted.

Of particular concern, he said, is that the law could prevent professionals such as police and corrections officers from seeking treatment for fear they could lose their firearms—and their jobs.

Already, officers often pay for treatment with cash rather than going through insurance to avoid anyone knowing about their psychological issues, Dr. Neblung said.

Under the law, therapists and others must use their professional judgment to determine if a patient poses a real threat. A decision to not disclose a patient's violent statements wouldn't lead to punishment if it was made "reasonably and in good faith."

Dr. Neblung said the language is vague and the potential for sanctions must be studied.

—Laura Nahmias contributed to this article.