Leonardo da Vinci primarily used his left hand for writing and drawing. His left-handedness is visible in surviving notebooks and drawings, where his pen strokes and hatching often move in ways associated with a left-handed artist. He also frequently wrote from right to left in mirror script, a practice that reduced the chance of smudging wet ink as his hand moved across the page.
Leonardo’s left-handedness was not merely a modern guess. Renaissance accounts documented it, and his manuscripts preserve abundant evidence of his writing habits. His reversed handwriting was once interpreted as a secret code, but it was more likely a practical consequence of writing with the left hand, combined with his natural preference for mirror writing.
The answer is sometimes complicated by evidence that Leonardo could occasionally write conventionally with his right hand. Some modern analyses have even discussed aspects of ambidexterity. Those details do not change the standard answer to this question: his primary working hand was the left. “Both” or “ambidextrous” overstates the evidence, while “right hand” is contrary to the dominant historical and manuscript record.