Sungai Merbok is one of the major rivers in Kedah. Though much shorter than Sungai Muda, it has the widest estuary of all the rivers in the state. Sungai Merbok empties into the Straits of Malacca with Tanjung Dawai on the northern bank and Pantai Merdeka on the southern.
Sungai Merbok flows through one of the most important regions of Kedah, the Bujang Valley. It is here that one of the earliest known civilizations in the country came into being, and even today, evidence of this is still being excavated and studied.
There are many rivers flowing into Sungai Merbok on both sides of its banks. Among the more well known is Sungai Bujang. Other rivers include Sungai Pial, Sungai Baharu, Sungai Siakap, Sungai Gelam, Sungai Batu Larangan, Sungai Keluang, Sungai Che Lela, Sungai Kerapu, Sungai Langgar, Sungai Batu on the northern bank, and Sungai Kerisek, Sungai Derhaka Dalam, Sungai Derhaka Luar, Sungai Terus, Sungai Teluk Wang Besar, Sungai Pasir, Sungai Petani, Sungai Jagong, Sungai Tiang, Sungai Che Tat, Sungai Layar, Sungai Limau and Sungai Lalang.
The banks of Sungai Merbok is densely covered with mangrove swamps, particularly as you approach the lowlands and the estuary.