On the night of 24–25 July 2020, after 21:00 local time (CEST, 19:00 UTC), Zagreb, Croatia, was struck by a storm which produced one of the worst flash floods in the city's history.

The torrential streams of the mountain overlying Zagreb surged and overwhelmed the city's combined sewer system, causing flooding.

The effects were worst felt in the inner city, notably Ilica, Donji grad, and Trnje, though many suburbs were also affected, including Buzin, Črnomerec, Jankomir, and Maksimir.

Hrvatske vode, the mayor, and the city's water and sewer utility company all denied responsibility for the flood and faced criticism from the public.

[6] A particularly violent flash flood occurred in July 1651, when the Medveščak stream overflowed during a night-time storm, washing away numerous houses and trees in Potok Street and causing 52 deaths.

The terrain transitions abruptly from hills to impermeable highly urbanised river lowland, occasionally overwhelming the sewer system during heavy rainfall events.

[23][24][25] The European Storm Forecast Experiment issued a Level 2 alert for an area including Zagreb, warning of possibilities of large hail, severe wind gusts and tornadoes, and noting that "an excessive rainfall risk could arise over parts of Slovenia and Croatia during the overnight hours.

Hail was recorded in Istria, and in the afternoon a flash flood struck Senj, a coastal city south of Zagreb.

As the trough turned into a low, the storm system stalled over Zagreb around 21:45, causing heavy rainfall and a high incidence of lightning.

[20][25] Areas affected by flooding included Ilica street, British Square,[25][31][32] Donji grad district, Trnje, Stara Trešnjevka,[25] Rudeš,[33] Jazbina,[34] Črnomerec neighbourhood of Šestinski dol[35][36][37] Jankomir, and Buzin.

[38] In Črnomerec, Fraterščica Road was closed due to water damage, while drivers had to be helped out of stranded cars in numerous places.

[39] In the low-lying district of Trnje, local residents reported that the flood arrived after 22:00, blowing out manhole covers before entering homes.

The representatives cited lack of urgency due to low water in Sava, at 161 cm (5 ft 3 in) below the zero level, as the reason for not staffing the dam.

[17] The ViO director credited a new interceptor sewer in eastern Zagreb with saving the neighbourhoods of Trnava and Sesvete,[61] which are not connected to the treatment plant.

In response to a subsequent media inquiry, Hrvatske vode stated that they would have opened the gate during the flash flood, had they received such a request from ViO.

[22][65] The official of Hrvatske vode responsible for flood control in the upper course of Sava was removed from his position as a result of the controversy.

[40][67][68][69][70] He had returned to the dam control shed, where he encountered and engaged in conversation with several police officers who were investigating the break-in, confessing to them his actions of the previous night.

[74][78][79][80] Attorneys consulted by the Jutarnji list newspaper and RTL television network stated that citizens are within their rights to film police officers in similar situations.

[78][80] Ombudswoman of Croatia [hr] Lora Vidović announced her office was investigating the incident, but declined to comment on the legality of the arrest.

[84] Hrvatske vode reported the control building break-in to the police,[61] and in September charges were filed against Blagus for breach of peace and disrespecting an officer of the law.

Zagreb Fire Department conducted several hundred interventions, mainly pumping out water from homes, and rescuing people from stranded cars, and basement and ground floor apartments.

President Zoran Milanović, Prime Minister Andrej Plenković and Mayor Milan Bandić expressed their condolences for the deceased firefighter and his family.

[114] News reports covered the poor shape of storm drains in Zagreb, the maintenance of which had ceased after the earthquake, according to ViO director.

[121] Opposition politicians criticised Bandić's response, blaming the disaster on a lack of communication between the City and Hrvatske vode, and a long-term state of disrepair of utility services in Zagreb, citing frequent water and district heating outages, open sewers near the city centre, and sinkholes opening up on street surfaces.

[122][106][123][124] ViO announced a 2 billion kuna (€260 million) project,[17] and plans to retrofit Zagreb's stormwater drain system to prevent such floods in the future, though "warning"[111] that it would not be possible without cooperation with Hrvatske vode.

Filip Šovagović and Dubravko Mihanović wrote Zagreb 2020, a play about the calamities which struck the city in 2020, including the flood.

[133][134] The play premiered on 27 November 2020 at the Lauba House,[135] and is as of June 2021 performed at other venues by the staff of Gavella Drama Theatre, whose building awaits earthquake damage repairs.