It is both. But, the roots of Bonsai do emerge from the Chinese, and their version of Bonsai was Penjing. (The e in Penjing is pronounced like the u in urban) The art of it dates back to at least 800 years ago, when Buddhist monks from China had handed a miniature plant in a pot to the Japanese as a gift for a certain good deed they had done. The Japanese had shaped, pruned wired and dramatically changed Penjing over the years through culture and inspiration of the people.
To answer your question in a simple version, it is both Chinese and Japanese. Bonsai is quite different in a way to the Penjing that we know today, and thorough out Bonsai have emerged new styles of miniature trees, and new techniques, but this wouldn't have been made possible without Penjing.