Are William Henry Harrison and Benjamin Harrison related?

Yes, William Henry Harrison and Benjamin Harrison are related. William Henry Harrison is the grandfather of Benjamin Harrison.
William Henry Harrison was the ninth president of the United States, and Benjamin Harrison was the twenty-third president. Benjamin Harrison's father, John Scott Harrison, was William Henry Harrison's son. John Scott Harrison was a congressman representing the state of Ohio, so Benjamin Harrison came from a long lineage of political forebears.
William Henry Harrison was voted in as president in 1840, while his grandson was elected to the same office in 1888. They are the only grandfather-grandson pair ever to have become presidents of the United States. William Henry Harrison is also notable for having had the shortest presidency in US history, lasting only 31 days before he died of illness.