Lake Peipsi-Pihkva, sometimes also called Peipus (; (Chudskoe-Pskovskoe), ) is the biggest transboundary and fourth largest fresh water lake in Europe, on the border between Estonia (part of European Union) and Russia.

The lake is fifth largest in Europe after Lake Ladoga and Lake Onega in Russia north of St. Petersburg, Lake Vänern in Sweden and Lake Saimaa in Finland.

Lake Peipsi-Pihkva is a remnant of a bigger body of water which existed in this area during an Ice Age. It covers 3,500 km², and has an average depth of 7 m, the deepest point being 15 m. The lake has several islands and consists of 3 parts:

• Lake Peipsi/Chudskoe (, ) is the Northern part of the lake. It covers 2670 km².

• Lake Pihkva/Pskovskoe (, ) is the Southern part of the lake. It covers 710 km².

• Lake Lämmijärv/Teploe (, ) is the sound connecting both parts of the lake. It covers 170 km².

The lake is used for fishing and recreation, but suffered from some environmental degradation from Soviet era agriculture.
Some 30 rivers and streams discharge into the Lake Peipus. Largest rivers are Emajõgi and Velikaya River. The lake is drained by Narva River.

In 1242 it was the site of the Battle of Lake Peipus (known in Russia as Battle on the Ice) between the Teutonic Knights and Novgorodians under Alexander Nevski.