|Location||Jaffna District, Sri Lanka|
|Native name||வடமராட்சி கடல்நீரேரி|
|Primary outflows||Indian Ocean|
|Surface area||77.87 square kilometres (30.07 sq mi)|
|Max. depth||2 metres (6.6 ft)|
|Surface elevation||Sea level|
Vadamarachchi lagoon (Tamil: வடமராட்சி கடல்நீரேரி) is a lagoon in Jaffna District, northern Sri Lanka. The lagoon is sometimes referred to as Thondamannar lagoon. The lagoon separates the Vadamarachchi region from the Valikamam and Thenmarachchi regions.
The lagoon is connected to the Indian Ocean by a narrow channel to the north, near the town of Thondamannar. The lagoon's water is brackish to saline. There is a sluice gate at Thondamannar to prevent sea water entering the lagoon.
The lagoon has extensive mudflats, seagrass beds and mangrove swamps, particularly Avicennia. The lagoon attracts a wide variety of water birds including American flamingoes, ducks, gulls, terns and other shorebirds.