Two men and one woman were killed by sea mines while swimming in Odesa, according to Ukrainian media.
A local official confirmed the three had been killed by “explosive devices”, at beaches close to Zatoka, where recreational swimming is banned.
The Black Sea has long been a popular holiday destination in Ukraine, but many of its beaches have been deemed unsafe since Russia’s full scale invasion.
Officials have urged holiday goers not to swim in prohibited waters.
Witnesses told local outlet Dumskaya that the explosions happened at 11:30 (09:30 BST) on Sunday between Karolino-Buhaz and Zatoka.
“All of them have been killed by explosive devices while swimming in areas prohibited for recreation,” regional governor Oleh Kiper confirmed.
“This once again proves that being in unchecked waters is fatally dangerous.”
Police say they have not yet confirmed the identity of the swimmers, and warned visitors “not to neglect safety measures”.
“It has been previously determined that three vacationers – a woman and two men – died while swimming as a result of two explosions of unknown objects. The identities of the deceased are being established,” the police report states.
Thirty two areas are safe for swimming, with 30 of these located in Odesa, according to authorities.