It is. It's considered both string and percussion because of the mallets that strike the inside of the piano to make sound (percussion), and the strings inside of the piano (strings).

The piano is a percussion instrument because it has hammers inside it which strike the strings. A percussion instrument is one where something is hit to make a sound.

The piano is a percussion instrument because it has hammers inside it which strike the strings. A percussion instrument is one where something is hit to make a sound.

The piano is a percussion instrument

A piano can be classified as a chordophone and a percussion instrument.