Oswald Achenbach (2 February 1827 – 1 February 1905) was a German landscape painter.

Born in Düsseldorf, he received his art education from his brother, Andreas Achenbach. His landscapes generally dwell on the rich and glowing effects of color which drew him to the Bay of Naples and the neighborhood of Rome. He also painted in the Bavarian Alps and Switzerland. From 1863 to 1872, he was professor of landscape painting at the Düsseldorf School. His conception of nature was more ideal than that of his brother. He died in Düsseldorf of an inflammation of the lungs. He is represented at most of the important German galleries of modern art, and many of his paintings are in the United States.