The Saint Petersburg Flood Prevention Facility Complex (Russian: Ко́мплекс защи́тных сооруже́ний Санкт-Петербу́рга от наводне́ний, romanized: kómpleks zashchítnykh sooruzhéniy Sankt-Peterbúrga ot navodnéniy), unofficially the Saint Petersburg Dam, is a 25 km (16 mi) long complex of dams for flood control near Saint Petersburg, Russia. The dam extends from Lomonosov northward to Kotlin Island (and the city of Kronstadt), then turns east toward Cape Lisiy Nos near Sestroretsk.

The complex is intended to protect Saint Petersburg from storm surges by separating the Neva Bay from the rest of the Gulf of Finland. The dam's tunnel also serves as the final leg of the Saint Petersburg Ring Road. The northern and southern parts of the dam act like two giant bridges, providing access from the mainland to Kotlin Island and Kronstadt.

Historically, storm surges from the gulf have caused over 300 floods of varying severity within the city, some with devastating effects. The dam has the capability to protect the city from water rising up to 5 m (16 ft). Its first use to hold back the incoming Baltic water into Neva Bay on 28 November 2011 resulted in a water rise of 1.3 meters above sea level (masl), below the flood level of 1.6 masl according to the Baltic system of coordinates. This prevented what would have been the 309th flood in the recorded history of the city, saving some 1.3 billion roubles of projected damage.

The construction of the flood prevention complex started in 1978 and became one of the longest construction projects in Russia. After a protracted halt in the 1990s and early 2000s, construction was resumed in 2005 due to the intervention of Russia's President Vladimir Putin, a native of Saint Petersburg. Putin finally inaugurated the finished complex in 2011, when all the facilities at the southern part of the dam were completed, along with the 1.2 km (0.75 mi) long underwater roadway tunnel below the main southern lock, the longest undersea tunnel in Russia.

Over 30 water purification installations are placed around the dam, part of a larger program to clean the water in the Neva Bay.

The dam spans the Gulf of Finland, with the island of Kotlin (Kronstadt) at its centre. It extends for 25.4 km (15.8 mi) and stands 8 m (26 ft) above sea level. It incorporates two large openings for shipping, which can be closed when floods threaten. Construction began in 1979 but stalled in the Russian political and economic upheavals of the 1990s; it resumed many years later and finally ended in 2011. The main benefit that most people cite^{[citation needed]} is not flood control but rather improved traffic flow, as the dam completes the Saint Petersburg Ring Road.

Saint Petersburg suffers from frequent floods (more than 340 in recorded history), some being natural disasters. It is situated on drained marshlands, isles and lowlands in the estuary of the Neva River, where flooding is common. Flow from Lake Ladoga is significant and the Neva's current is rapid, but flooding is generally caused by water backing up the Neva from its outlet, the Gulf of Finland. Most rivers flood in periods of exceptionally high flow, but the Neva typically floods in late autumn.

In the early literature, high winds from the Gulf of Finland were often cited as the cause of Neva flooding, but scientists now understand the more complex hydrometeorological chain of events behind it. A low-pressure region in the North Atlantic moves onshore, giving rise to cyclonic lows on the Baltic Sea. The low pressure of the cyclone draws greater-than-normal quantities of water into the virtually landlocked Baltic. As the cyclone continues inland, long low-frequency seiche waves are established in the Baltic. When the waves reach the narrow and shallow Neva Bay, they become much higher, ultimately breaching the Neva embankments.