The decision came as Da Nang recorded 24 new coronavirus cases since Friday after more than a month without detecting new community transmissions. Only six of these have been tagged as Covid-19 patients by the Health Ministry.
City leaders said the pandemic situation "is turning complicated" and that the swimming ban would remain in place until further notice.
On June 9, city authorities reopened public beaches and allowed restaurants and eateries to resume business with safeguards after the pandemic situation was put under control.
However, a 31-year-old man was confirmed positive for the virus Friday, ending its 31-day streak without community transmissions. Following contact tracing and mass testing, the city recorded 23 more infections linked to the man.
Da Nang has recorded 182 infections since April 27 as the fourth coronavirus wave hit the country.
The popular tourist city has a coastline of 30 kilometers, which includes many of Vietnam's most beautiful beaches like My Khe, Non Nuoc and Nam O.