The father of a man on trial for attempted murder told jurors Monday that letting his bipolar son live with the Upper West Side psychiatrist cousin who allegedly put him up to the crime might have been the worst mistake he ever made.

James Nolan encouraged his son Jacob Nolan, 23, to move in with Dr. Pamela Buchbinder in 2012.

It “might have been the worst decision I ever made,” he told jurors in Manhattan Supreme Court.

Jacob Nolan allegedly tried to kill Buchbinder’s ex, Michael Weiss, with a sledgehammer in November 2012. The defense argued the doctor devised the plot to cash in on Weiss’ insurance policy.