There is a potential to formulate cost effective aquafeeds using seaweeds in the littoral zone of the coastal belt of Sri Lanka. This study was designed to evaluate the effect of seaweed supplemented diets on the growth of Koi carp. The experimental set up consisted of a control (C) and two experimental treatments (T<sub>1</sub> and T<sub>2</sub>). The control diet (without seaweed) and the two test diets, of which 30% of the weight of control diet was replaced by <em>Ulva lactuca </em>and <em>Sargassum cinereum </em>were fed to Koi carps in C, T<sub>1</sub> and T<sub>2</sub> respectively. Proximate analyses were carried out for both ingredients, and the diets were prepared following standard methods. Koi carp in fry stage belonging to the same cohort (weight 2.20±0.08 g, length 3.89±0.04 cm) were assigned to C, T<sub>1</sub> and T<sub>2</sub> in fiber tanks filled with 150L of water (n=20 per tank) with four replicates each. The fish were fed 5% of their body weight twice a day (09.00h and 15.00h) for 14 weeks. The length and weight measurements were recorded once in two weeks. Fish performance was compared using Body Weight Gain (BWG), Daily Growth Rate (DGR), Relative Growth Rate (RGR), Feed Conversion Ratio (FCR), Feed Efficiency (FE), Fulton’s Condition Factor and survival rates. At the end 14<sup>th</sup> week of the experiment, mean values of the weight, length, BWG, DGR showed a significant difference (<em>p</em><0.05) among treatments. Tukey test revealed that FCR of the T<sub>2</sub> and T<sub>1</sub> were significantly different (<em>p</em><0.05) from the Control and FE of T2 was significantly different (<em>p</em><0.05) from the C. Condition Factor and water quality parameters did not show a significant difference (<em>p</em>>0.05) among treatments. There was a 100% survival rate in all groups. Aquafeeds supplemented with seaweeds showed promising results in terms of increasing growth rates of young Koi carps.