Giannitsa Lake (Greek: Λίμνη Γιαννιτσών), also known as Loudias Lake (Greek: Λίμνη Λουδία) is a former post-glacial lake in Central Macedonia, Greece, south of the town of Giannitsa and north of Gidas (later renamed Alexandreia).

[1] Shallow, swampy, and variable-sized, it was drained from 1928 to 1932 and became agricultural land.

It was fed by the Loudias River and the plain of Roumlouki stretched to its south.

The Lake played a major role in the Macedonian Struggle between the Greeks and Bulgarians, as it provided hiding places for the armed bands of both sides.

The conflict for control of the Lake is the central historical event of Penelope Delta's 1937 novel The Secrets of the Swamp.