The Merchimden () is a river in Yakutia (Sakha Republic), Russia. It is a tributary of the Olenyok with a length of and a drainage basin area of .
The river flows north of the Arctic Circle across a lonely, desolate area of Olenyoksky District devoid of settlements.
The Merchimden originates in the eastern fringes of the Central Siberian Plateau, very near the sources of the east-flowing Motorchuna. The river flows initially to the northeast, then roughly northwards all along its course, parallel to the Olenyok flowing in the same direction a little further to the west. In its last stretch it meanders very strongly. Finally it meets the right bank of the Olenyok from its mouth. The confluence is just a little downstream from a very strong eastward bend of the Olenyok. There are a few small lakes just east of the confluence.
The river is frozen between the first half of October and early June.
The main tributaries of the Merchimden are the long Khayalaakh-Yurekh (Ã¥ðùðûððà-îÃÂÃÂà) and the long Magan-Khayalaakh (ÃÂðóðý-Ã¥ðùðûððà) from the left, as well as the long Pastaakh-Yurekh (ÃÂððÃÂÃÂððà-îÃÂÃÂà) and the long Ulakhan-Kurung-Yurekh (ãûðàðý-ÃÂÃÂÃÂÃÂýó-îÃÂÃÂà) from the right.