Chiu told a crowd of about 100 people at the rally that he wants to make sure San Francisco remains "a beacon to the rest of the world."
Chiu is the latest entrant in an already crowded mayoral contest that will be decided in November.
State Sen. Leland Yee, City Attorney Dennis Herrera, and former supervisors Bevan Dufty, Michaela Alioto-Pier and Tony Hall are among the other candidates in the running.
"It's a wide field," Chiu said. "San Francisco is blessed to have so many great candidates."
He said he has separated himself from the other candidates by helping to foster change in what was said was often a "dysfunctional" relationship between the board and the mayor.
Former Mayor Gavin Newsom frequently sparred with supervisors, but Chiu said he has helped change the tone by encouraging collaboration between the board and new Mayor Ed Lee.
Chiu served as mayor for one day in January. In accordance with the city charter, as president of the board, Chiu held the position after Newsom was elected the state's lieutenant governor and before Lee was sworn in as interim mayor.
Lee, formerly the city administrator, is expected to serve out the year as mayor and then return to his old job.
After his announcement this morning, Chiu went into City Hall to officially file his declaration of intent to run at the elections office.
The race will be decided by ranked-choice voting, a system that Chiu said he supports.
"I think it's a system that fosters positive campaigning," he said.
Supervisor Eric Mar attended today's rally, saying he supports Chiu for mayor because "he has the ability to bring communities together...and find common ground."