On the Democratic side, Alderman Joyce Craig will look to become the city's first female mayor. Patrick Arnold, a former alderman who lost to Gatsas by six percentage points in 2013, is running again. And businessman Jawed Alibaba Shaikh will also seek the city's top job.
Political analyst Dean Spiliotes says the three challengers are all using the same line of attack against Gatsas.
"The bigger picture here is, 'Is Mayor Gatsas doing enough to really build the kind of business climate you want to attract business, to keep the economy vibrant?' " Spiliotes said.
The election is nonpartisan, which means that the top two vote-getters in September's primary, no matter their party, will face off in November's general election.
Gatsas has been mentioned as a contender for statewide office in the past, but with this announcement, Spiliotes doesn't expect him to seek a higher post any time soon.