De Blasio, City Council member Steve Levin and others were outside the midtown Manhattan offices of the State University of New York, which is considering selling Long Island College Hospital in Brooklyn. The protest was partly organized by a health care workers union that has endorsed de Blasio for mayor.
Both de Blasio, who is currently the city's Public Advocate, and Levin were cited on a disorderly conduct charge.
"The police were very respectful" while making the arrests, de Blasio said.
De Blasio has urged Gov. Andrew Cuomo to intervene on the side of keeping the hospital open.
"It's not about my campaign. It's about saving hospitals," he said after he emerged from the police precinct where he and others were given a summons for an Aug. 20 court appearance. "And it's time to save these hospitals."
De Blasio has been arrested four times previously, including at a protest over the closing of a Brooklyn firehouse in 2003, when he was a city councilman. Actor Steve Buscemi, a former city firefighter himself, also was arrested at the same rally.
Several Democrats and Republicans are running to succeed Mayor Michael Bloomberg, who cannot seek re-election. The primaries are Sept. 10. The general election is Nov. 5.