Hard sciences use the scientific method to draw conclusions about the natural world. The scientific method involves observing, measuring, gathering data, testing hypotheses, experimenting, and developing theories. Psychology employs every facet of the scientific method, and must be considered a hard science.
Simply using the scientific method does not make psychology a science. Science observes concrete cause and effect processes which reveal truths about how the world works. Psychology relies on observing human behaviors which are unpredictable, yielding conclusions which are abstract at best.
[P1] Psychology is a science because it employs the scientific method.
Rejecting the premises
[Rejecting P1] Using the scientific method alone isn't enough to classify something as a science.