"The bills I'm vetoing today were passed without the level of collaboration that I think we need and, more importantly, that the city expects from us", Jenny Durkan said at a video conference.
Durkan, a Democrat, added she was optimistic that she and the city council would be able to compromise on budget cuts that would not put public safety at risk.
The revamped budget seeks to cut pay for some of the senior police officers and do away with an outreach team for the homeless.
The city council may override the mayoral veto since the new budget was approved by a supermajority. The city's legislative body is on a recess and will come back in a few weeks.
The calls to defund, reform, or even dismantle the police started to emerge in the wake of the nationwide demonstrations following the death of George Floyd in Minneapolis. During the rallies, a group of protesters created the Capitol Hill Autonomous Zone in Seattle, Washington, occupying several blocks for over two weeks and proclaiming them a "police-free" area.