Yuma Mayor Doug Nicholls issued a proclamation of emergency, saying the continued mass release of migrants directly into his border community had become an "imminent threat" to life and property in the area.
The city lacks the resources it needs to handle the migrants, he said.
Yuma is the first border city in the U.S. to declare a state of emergency as part of its response to the latest surge in the number of migrant families reaching the U.S.-Mexico border to seek asylum.
"Today is a day that we had talked about three weeks ago, hoping never to get to," Nicholls said at a news conference on Tuesday.
With a "heavy heart," the mayor said he signed the document as a means to seek resources and to protect residents and vulnerable migrants.
