The river Kirtankhola has great importance in southern part of Bangladesh due to its necessity in transportations, municipal and agricultural purposes. But the ecological status and impacts of water quality on phytoplankton diversity were undiscovered. To cover the maximum area of the river, water samples were collected from the 6 locations (ST) covering about 13 km. Some parameters were measured immediately after sampling by portable devices and others done in laboratory following standard methods. The overall quality of the water was found good according to water quality guidelines of World Health Organization, although the water of port area showed comparatively high SO42- and CO3- values which attracts attention for future monitoring. ST 6 showed comparatively more alkaline water (pH 8.45) along with highest EC, TDS, NO3-, NH4+ and DIP values. Besides, 53 phytoplankton taxa belonging to Cyanophyceae (6), Chlorophyceae (10), Euglenophyceae (12) and Bacillariophyceae (25) were identified from the river. Scenedesmus acuminatus was found commonly in five stations except ST 1, whereas Euglena chlamydophora (ST 1), Gomphonema olivaceum (ST 3) and Cymbella parva (ST 5) found rarely. Highest diversity was counted in ST 6. Moreover, Chlorophyceae and Bacillariophyceae have positive correlation with several physical parameters, whereas Cyanophyceae negatively correlated. Euglenophyceae has both negative and positive correlations with some physico-chemical parameters. However, this was first attempt to measure diversity of primary producers and their correlations with water quality of Kirtankhola River.