An experiment was conducted to determine the effectiveness of potassium in mitigating drought stress and improving yield in lowland rice ecosystems at Sokoine University of Agriculture Morogoro Tanzania. The main plot constituted three soil moisture levels viz. flooding, 20 and 40 kPa soil moisture tensions. Subplot factor constituted three lowland rice varieties viz. SARO, NERICA4 and SUPA while potassium levels were 0,50 and 100 kg K ha-1 for field experiment and 0, 25 and 50 mg K kg-1 of soil for pot experiment as sub-subplot factor. The experimental design was split-split plot in randomized complete block design with three replicates. The results showed that rice plants subjected to saturation soil moisture level had higher panicle length, number of spikelets and number of grains panicle-1, percentage spikelets fertility, grain yield, root dry matter, straw dry matter and total dry matter yield. However, unfilled grains increased as soil moisture tension increased. The results indicated that the studied rice varieties differed on plant height, tiller number, shoot dry matter, grain yield, root dry matter and straw dry matter. SARO variety had the highest grain yield followed by NERICA 4, while SUPA revealed the least grain yield for pot and field experiment. Potassium nutrition at 50 kg K ha-1 enhanced yield and yield components of rice varieties under saturation soil moisture level and drought conditions. Grain yield was significantly and positively correlated with number of grains panicle-1 and percentage spikelets fertility. The study confirmed that potassium alleviated drought stress and improved yield and yield components of rice varieties. However, saturation soil moisture level should be maintained under lowland rice ecosystem for high grain yield.