Isaac Newton was a brilliant scientist who made significant contributions to several branches of science, but he is most famously associated with physics and mathematics.

Here's a breakdown:

* Physics: He is considered one of the most influential scientists of all time for his groundbreaking work on classical mechanics, which describes the motion of objects. His laws of motion and law of universal gravitation revolutionized our understanding of how the universe works. He also made important contributions to optics through his work on light and color.

* Mathematics: Newton is also credited with developing calculus, a powerful tool for understanding change and motion. His work in mathematics was integral to his discoveries in physics.

While he is most famous for his work in physics and mathematics, Newton also made contributions to astronomy, alchemy, and theology.

So, while there isn't one single "branch of science" that Isaac Newton belonged to, he is best known for his contributions to physics and mathematics.