Probably, based on structural features and fossil evidence, though some scientists studying DNA evidence suspect otherwise.
There are studies, primarily of mitochondrial genes, suggesting that guinea pigs might not be rodents, but might belong to their own separate order. (Mitochondrial DNA is carried by the structures in cells that act as cellular power plants. It is inherited in a manner different from ordinary DNA.)
However, most of these studies were based on a limited sample of genes in a limited number of orders. More gene sequences should be studied for more mammals before we can be sure of the answer.
Another recent analysis showed that by using the same data as some of these studies, but employing another commonly accepted model of gene evolution, guinea pigs can be placed in the rodent group.
Finally, the traditional kind of evidence in mammal evolution is very strong that guinea pigs belong in the order Rodentia, both from morphology (structural features such as the head and the development of the gnawing teeth) and from paleontology (fossils from Central Asia).
