The massive party was held near Ritzville, in rural Adams County, on Nov. 7, officials said.
By Monday, neighboring Grant County had traced 17 cases of COVID-19 to the event, "with more being added daily," local health officials said in a news release. On Tuesday, the number of cases connected to the wedding was approaching 40, the Grant County Health District confirmed to ABC News.
A long-term care facility and a school district also have outbreaks tied directly to, or through contact with, wedding participants, the health district said.
Grant County health officials are asking attendees of the wedding to get tested for COVID-19 and to self-quarantine through Saturday. The health district said it is trying to notify attendees about possible exposure, "but with more than 300 people attending the wedding from many communities, local health jurisdictions are unlikely to reach them all."
Under current restrictions, wedding ceremonies in the state are limited to 30 people, the health district said.
The site of the wedding was not identified, but described as a "private location." Grant County residents threw the party, officials said.
ABC News' Leah La Rosa contributed to this report.