The child was pulled off the dock by the otter and briefly dragged underwater during the incident, according to the Washington Department of Fish and Wildlife, ABC News reported.
The rare attack occurred Thursday morning at a marina in Bremerton, located on the Puget Sound across from Seattle, WDFW said.
"The victim's mother reported that a river otter pulled her young child into the water as they were walking down the dock," the department said in a press release. "The child was dragged underwater, and after a few moments, resurfaced."
The mother was able to pull the child out of the water as the otter continued to attack, the department said. The animal continued to pursue them as they left the dock, it added.
The child sustained scratches and bites to the top of the head, face and legs, and the mother was bitten in the arm during the attack, WDFW said. The child was treated at a local hospital for minor injuries. The age of the child was not available at this time, a department spokesperson said.
"We are grateful the victim only sustained minor injuries, due to the mother's quick actions and child's resiliency," WDFW Sgt. Ken Balazs said in a statement. "We would also like to thank the Port of Bremerton for their quick coordination and communication to their marina tenants."
The mother and child were not publicly identified.
State wildlife officials contacted the U.S. Department of Agriculture Wildlife Services to trap and "lethally remove" river otters from Bremerton Marina, WDFW said. One river otter was trapped at the scene and transported to a lab for further evaluation, including testing for rabies, the department said in an update on Friday.
River otters are fairly common throughout Washington and are often found in fresh, brackish or saltwater habitats.
There have been six documented human-river otter incidents in Washington in the last decade, according to WDFW.
"Although encounters with river otters are rare, they can be territorial and, like any wildlife, are inherently unpredictable," WDFW said.