The Pacific White-sided Dolphin (Lagenorhynchus obliquidens) is a very active dolphin found in the cool to temperate waters of the North Pacific Ocean.

The Pacific White-sided Dolphin was named by Theodore Gill in 1865. The Pacific White-sided Dolphin is extremely similar morphologically to the Dusky Dolphin, which is found in the southern part of the Pacific. Some researchers have suggested that they might form a single species. Recent genetic work by Frank Cipriano rejects this hypothesis and suggests the two species diverged about two million years ago.

Though the Dusky and Pacific White-sided Dolphins are traditionally placed in the genus Lagenorhynchus, recent molecular analyses indicate that they are actually more closely related to the dolphins of the genus Cephalorhynchus. If true, this would mean that these two species must either be transferred to Cephalorhynchus or be given a new genus of their own.

The Pacific White-sided Dolphin has three tones of color. The chin, throat and belly are creamy white. The beak, flippers, back, and dorsal fin are a dark gray. There are light gray patches on the sides and a further light gray stripe running from above the eye to below the dorsal fin where it thickens along the tail stock. A dark gray ring surrounds the eyes.

The species is of average size for an oceanic dolphin, weighing up to 150 kg in females and 200 kg in males and growing up to 2.5 m (male) and 2.3 m (female) in length. Pacific White-sided Dolphins tend to be larger than Dusky Dolphins. Females reach maturity at 7 years. The gestation period is one year. Individuals can live for up to 40 years or more.

The Pacific White-sided Dolphin is extremely active and mixes with many of the other cetacean species to be found in the north Pacific. It also readily approaches boats and bow-rides. Large groups are common; on average 90 individuals per group but supergroups of more than 3,000 have been recorded. Prey is mainly hake, anchovies, squid, herring, salmon and cod.

The range of the Pacific White-sided Dolphin runs right in a great arc across the cool to temperate waters of the north Pacific. Sightings go no further south than the South China Sea on the western side and the Baja California peninsula on the eastern. Populations may also be found in the Sea of Japan and the Sea of Okhotsk. In the northern part of the range, some individuals may be found in the Bering Sea. The dolphins appear to follow some sort of migratory pattern — on the eastern side they are most abundant off the California shore in winter, but further north (Oregon, Washington) in summer. Their preference for off-shore deep waters appears to be year-round.

The total population may be as many as 1 million. However such is the tendency of Pacific White-sided Dolphins to approach boats from some distance away, it is harder than usual to obtain precise estimates via sampling.

Until the United Nations imposed a ban on certain nets in 1993, many Pacific White-sided Dolphins were killed in drift nets. One researcher estimated somewhere 50,000–89,000 individuals were killed in the twelve years to 1990. Some animals are still killed each year by Japanese hunting drives, though these are not expected to cause a threat to the species at the present time.

Although overshadowed in popularity by Bottlenose Dolphins, Pacific White-sided Dolphins are also a part of some marine theme park shows.

Venus Among the Fishes is a book, written by Elizabeth Hall, with a female Pacific White-sided dolphin as its protagonist.