Albert Einstein did not discover the third law of motion. That credit goes to Sir Isaac Newton, who formulated the three laws of motion in his book *Philosophiæ Naturalis Principia Mathematica* published in 1687.

The third law of motion states that for every action, there is an equal and opposite reaction.

While Einstein made groundbreaking discoveries in physics, they were primarily in the realm of relativity and not classical mechanics, which is where Newton's laws of motion belong.